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