A slave(girl) lock.

I got this idea, as I was considering the possibilities of having a masochist 'available', without letting the masochist i.e. masturbate.

Unfortunately I only have the drawing, you see here, but I'll explain it, to you, to let you get the idea. *S*

The basic concept is, that the device is having a collar, at the top, and that the bottom end, is attached to the masochist's body via the buttplug, which in fact is the first part of the device to be mounted.
The vertical pipe is intended to go down the back of the masochist, more or less following the spine. The elbow-shackles and the wrist-shackles will hold the arms firmly secured behind the masochist's body, down his/her back, leaving the front side of the masochist 'out into the open'. freely available to anyone, except the masochist trapped inside the device.

1 Neck-collar

Here at the top of the device, I start out with a collar, intended to be locked around the neck of the masochist.

I guess that some arrangement with a padlock could come in handy.

I have outlined the collar as 'made of steel', but in principle, you can choose just about any material which you prefer.
2 Elbow shackles

The elbow shackle must grip around the arms, just below the elbows, in order to hold the underarms tightly shackled from here, down the masochist's back, following the spine.

If needed, the elbow shackles can be made so, that they are somewhat apart from one another, if your masochist is unable to assemble the elbows on the back, but the 'real deal' would be, to hold the elbows tightly cinched together, through a shackle around either arm. If the elbow shackles are placed at the correct distance from the shoulders, it will be impossible to escape from them, as the upper arm bone cannot be bent.

If you place them, at the right height, they do actually not need to be 'real tight', although your masochist might prefer them to be, as this will 'hold' the arms in a more (or less) pleasant manner.
3 Wrist-shackles

The wrist shackles might be a set of handcuffs, attached to the pipe, at the right distance from the elbow shackles.

You can of course pick just about any material for these, as well as for the others, bur here I have sketched them in steel, probably locked i.e. with two padlocks. If they are placed at the correct distance from the elbow shackles, you will have your masochist's arms trapped, no matter what, as I haven't seen a masochist who were able to bend the bones inside their arms.

If you'd like to, you can actually let the elbow shackles and the wrist shackles consist of rings, which will allow for some movement of the arms and shoulders, then you'll just need to place the wrist shackles somewhat above the wrists (I guess that 4" would do the trick), again, as the rigidity of the bones will limit how much the masochist will be able to move the arms up and down, as all the movement needs to be in the shoulders.
4 Butt plug

The butt plug is actually the first thing to be attached to the masochist, unless you've chosen to use the idea with rings around the elbows and wrists.

Having inserted the butt plug into your masochist's rectum, you then lock the neck collar, and by now your masochist is actually trapped in the device, unless your masochist is extremely flexible.

If your masochist want to 'wiggle out', he/she will need to bend the back forwards, to such a degree, that the bending 'shortens' the length of the back (between the neck collar, and the butt plug) to allow for the butt plug to 'slide out', as the pipe running down your masochist's back will stop any attempt to shorten this distance, by bending the back in the usual way. And if they try that, they'll just bury the butt plug in even deeper..*G*

In case you'd like to be able to use this contraption on more than one masochist, you'll need to make an adjustment to the design, as you'll need to mount the neck collar with a bracket, which will allow it to slide up and down the steel pipe, at least until you turn the locking screw(s), tightening the bracket to the pipe, i.e. by the use of a unbrako screw. (As shown in the drawing).
In the same manner, the wrist shackles and the elbow shackles must be mounted on brackets, in order to let you re-position them, according to the body of the one, who's going to wear the contraption.
Furthermore, you'll need to make the vertical pipe long enough, to cover even a baseball player, making sure, that the top of the pipe, is connected to the neck collar in such a way, that it allows for the pipe to pass upwards, behind the back of the head. (As shown in the drawing).
Using this modification, you'll furthermore have the option, to let the top of the pipe have a ring, which can be tied to another ring, i.e. in the ceiling, if you'd like to limit your masochist's ability to walk around. However, if you tie somebody up, in this manner, you'll of course need to take care, that your masochists will not 'hang themselves' i.e. in case of a good orgasm, where the knees might become 'somewhat buckly' afterwards.

Talking of alternatives, it would be possible, to let the butt plug be a bi-polar electro-stimulation butt plug, which in turn would give you a 'remotely controlled' masochist, according to what kind of electro stimulation device you use.

By the way, did I forget to mention, that a female masochist, with her arms trapped in the way this contraption will do, will also be tightening her chest muscles, in such a way, that she'll be presenting her breasts in the most delightful manner?

It should be obvious, but now I'll mention it anyhow: a masochist, inside a contraption like this cannot and may not be left alone, chained/tied to the ceiling or not..!