Overturned Y.

The explanation is quite simple. The overturned Y has a dildo-sleeve ( Like the ones you can get to change the 'cover' on a vibrator) on the one end, and a set of ankle-cuffs on the two other ends.

The 3 sticks are assembled by hinges, so that the dildo will move into her vagina, when she assembles her legs, and out when she's spreading them..

It's necessary to add a piece of string or chain between her ankles, to prevent her from spreading her legs so much, that the dildo 'drops out'.

I have had the opportunity to test the gizmo, and now I've finally succeeded getting photos of it...

The Y is made from 3 sticks,( i.e. 25 mm thick ); 2 hinges, a dildo-sleeve, 2+2 screw-eyes, a piece of line/chain, and 2 ankle cuffs.
1/ Cut two (identical) pieces of stick, reaching from the ankle, to a little above the knee.
2/ Mount the hinges on either stick, at the one end, and mount a screw eye at the other ends.
3/ Cut another piece of stick, so long, that it'll reach from the ends of the first two sticks, to her vagina + 15 centimeters, or more, if she likes it.
4/ round off the one end of the last piece of stick (which goes inside her vagina)
5/ Mount the hinges to the other end of the dildo-stick, one at either side.
6/Mount the remaining 2 screw eyes, on the inside of the two sticks to be attached to her ankles ( facing one another ).
7/ Pull the dildo-sleeve over the rounded end of the stick.
8/ Strap the ankle cuffs to her ankles, and let her stand with the feet so far apart, that she'll have about 5 cm of the dildo inside her vagina, when you attach the screw eyes at the 'bottom-ends' of the sticks to her ankle cuffs..
9/ Bind the line (chain) between the screw-eyes on the lower sticks, to prevent her from 'loosing' the dildo by spreading her legs. ( it needs to be rather tight ).
10/ Now you've secured that she'll be fucking herself whenever she's walking.

Now, when she's walking, the dildo will a): slide in and out of her vagina, and b) rotate inside her vagina...
Now, dress her up in a long skirt, and take her for a walk in the forest, shopping or...       

PS: wearing this contraption, she will not be able to sit down !

Version 2:

Version 2 doesn't actually differ that much from version 1, apart from the fact, that it's made with a 'harness' resembling a girdle.
However one important difference is, that this version will move the dildo UPWARDS when she's spreading her knees, and DOWNWARDS when she's holding her knees next to one another; in opposition to version 1.
This design give a few advantages:

1. She cannot 'loose' the dildo, if it's made to the correct length from the beginning.
2. She will decide how 'deep' she wants the dildo to penetrate her pussy, on each step. The further apart her knees are, the further she'll get penetrated.
3. This build can be hidden away underneath a just-below-the-knee length of skirt/dress, whereas the version 1 will need a floor-length skirt/dress, in order to hide the ankle straps.

And it comes, a male edition.

Well, finally I got myself pulled together, to try to make a male edition, of the overturned Y, which 'acts' on the cock, in stead of 'acting' inside the vagina.
It is - however - not an instant success, but I've got an idea, which might make it a success, and I'll try to get that into the real world, as soon as possible.

I've made a video, of the male overturned Y, which you'll find here: