U know, I actually successfully built a rather effective one in my teens.
First, i got a regular belt, a cardboard tube, some string, and that's it!
The cardboard tube is actually a toliet roll.
Steps:
1) Poke/Punch 3 holes on the tube, one on the top, and 2 on the bottom, on opposite
sides. That's two holes in front and one on the bottom.
2) I put on my belt around my waist tight enough for no-movement but not
uncomfortable.
3) Using a shorter string, i tied a knot at one end and threaded it through the bottom
hole. I then looped it around my belt and put it into the top hole, finishing off with a
knot to keep it in place.
4) With a longer string, i knotted it off at one end and threaded it through the bottom
hole. I then pulled it down between my legs so my penis was forced downwards and
looped it once around my leg. One leg only. For two legs, u need to make two bottom
behind loops. After looping it, i tied the other end to the shorter string.
Functions:
Belt: Obvious really
Tube: House the penis
Short string: Hold the tube up in case of an un-erection
Long String around legs: Keep the tube down
Materials may vary.
This is rather effective, as i wore it the whole day and nobody noticed.