Over-the-door-strap...

Sometimes you get into a situation (especially in older buildings*), that the ceilings cannot support a hook, or at least not a hook, which can carry a person.
One possibility could be to start searching for the timber witch support the ceiling, but this could prove to be a long search.
This solution is by no means the perfect one, but it will come at a reasonable cost, is easy to make and finally, it's possible to bring it along anywhere.

The strap itself is made from a 2' piece of cotton rim, a piece of metal pipe and 2 D-rings.
The metal pipe could just as well be i.e. a piece of wood, or just about anything else, witch will prevent the end of the strap from 'slipping over the door'.

The production is actually quite simple, but I'll walk you through it anyway:

1. Get a 2 foot long piece of i.e. cotton rim, and sew it at both ends, to keep it from fraying.
2. Find the middle of the rim, and sow the first D-ring into place here.
3. Approximately 4" from that, sew across the rim, through both layers.
4. Pull the second D-ring in, over one of the layers of rim, and sew it into place, along the seam from 3/.
5. Finally sew the 'stick' into place, as illustrated in the sketch below.

The sketch is to be interpreted like this:

• The red line is one end of the rim.
• The blue line is the other end of the rim.
• The back/gray circle is the 'stick'.
• The green lines indicate where to sew.
• The rim must (most certainly) have a snug fit around the stick in the real world, but here I've opened the thing up, to let you see what goes on where and how.

The strap over the top of a door, having the stick on one side, and the D-rings on the other side of the door.

Here you see the strap, placed over the top of the door, from the 'bondage side' of the door.

Here you see the strap, from 'the other side' of the door (the side of the door, facing away from the room, in which you intend to anchor something to the strap).

Please notice, that the strap is best used, if the door you are using is 'opening away from you' - seen from the 'bondage side' of the door.

The logic here is, that if you pull the door in the direction in which the door will normally open, it is only the door's closing mechanism, which is keeping the strap in place.
Many doors are (unfortunately) build from 2 thin layers of veneer, with almost nothing in-between them, making them rather 'bendable'. This will, if you pull the top of the door towards the 'opening side' result in the door bending, possibly enough to let the stick slip though (shown with a red line representing the strap).

If you - being a smart person - choose to stress the door towards the 'closing side', the door will 'lock' inside the doorframe, keeping the strap securely in place. This is illustrated, usiong a green line to represent the strap - and will give you the advantage that you can let the strap carry a far heavier load.

DO make sure, that you keep the stress on the door, and the strap within safe limits, and most important be aware of the fact, that if someone open the door, the strap will not stay in place..!