The form started with floral reservoir foam. Very easy to shape. I used a couple different saws for the basic shaping. To round out the seat area I built it up with children's modeling clay. Yes I actually sat in it.
No need to let the clay dry. Just laid the glass mat (8 layers) and epoxy on it. I will add that I have VERY little experience with fiberglass. What's up with the non-woven mat? That stuff just falls apart!
Next I wrapped with stretch wrap and electrical tape to make sure that the edges of the mat conformed to the foam mold. I even sat on it again when it started to set up.
After it set the foam is easily broken out and the clay washes out with water.
Next I trimmed the edges of the base plate, mostly with a saber saw. Here's were I would have done it differently if I had to again: I left some curved edge around the back of the seat. I would trim this flush to make a sleeker / flatter look and make it easier to stretch the leather.
Then came two layers of half inch yoga mat with contact cement. A single layer may look better but I'm all about comfort.
The mounting hardware is 3/8" carriage bolts/washers/nuts and a piece of steel flat bar I had laying around. I'll use a bobber style seat mount hinge so I may not need the flat bar.
Next came the hard part. I had zero experience with leather. I had the preconceived notion that I would stretch the cased leather in place, dry it, cut it dye it and leisurely hand stitch it (just like the youtube videos). I soon realized that the ONLY way to hold the cover in place was to stitch it on.
For anyone wanting to try something like this, the Tandy hand stitcher is AWFUL! I got one at Michael's with a bobbin inside the handle that is MUCH better.