Actually it is remarkably easy to make a simply chest harness. All you need is about 8-10 feet of 1" tubular webbing which can be purchased at a climbing store for about $0.25-$0.40 per foot. Then tie the ends together to make a big loop. (Technically a water knot should be used for maximum strength, but for your purposes just make sure you use a secure knot).
Then make an "X" with the loop and put your arms through the two sides of the "X", one arm on each side so the "X" is on your back. Then use a carabiner an clip it over the "X". To pull from the front just reverse it so the "X" is on the front, although this tends to slip off a litte more easily so keep your shoulders back.
Personally I use an 18' length double looped so it is more comfortable, but it really doesn't matter.
It is really simple and the picture will show it better than I can explain. To pull from the back just reverse the way you put it on. And the seat harness isn't necessary, so just use the chest harness shown in this picture: