I lost my retainer about 6 months ago. It was a clear invisalign retainer, and I really didn't want to pay 200 dollars to get a new one. So I made my own, and they work great.
The pictures show the finished product.
Notice that they are white, so they are only really good for night time, unless you want to look like a dork. Usually after 6 months you can only wear them at night anyways.
Step 1: Materials/ Tools needed
A glass cup
Around 5 grams of Shapelock plastic, aka friendly plastic, per retainer
The best place to get the plastic is by getting a 35 gram sample for 5 dollars here
Step 2: Melting and forming
Fill up the glass cup with water and put it in the microwave until it bubbles (around 2 min). Pour the shapelock plastic into the water and let it sit until it is all clear. Use a fork or something else to press the plastic together into a ball. Once it is totally clear, take it out with a fork and pull a chunk off of it, around 5 g. Then shape it into a half ring in the shape of your teeth. Make sure it is wide so you can fold it over the teeth. (See pic)
Do not try to press it onto your teeth yet. Once it is in a basic half ring shape, put it back into the water to let it soften again, then take it out and fold the edges in. (See pic)