A few years back I purchased the "cast a willy", I think from CastingWilly.com if I remember correctly. "Cast any size" it said. So I purchased it. Not true.
Staying hard is difficult but I managed to keep my dick inflated by using a 2" outside diameter rubber "O" ring. My dick is 8.5 " long by 6.75 in circumference.
Well, I made the mold ok but when it came to filling it with the supplied liquid rubber I discovered that there was not enough liquid to fill the dick mold. The cast of my dick came out really good. Each vein, cell, etc. showed up with exquisite detail. But because I did not have enough liquid rubber to fill the mold, my cast came out 2 inches short. It was 6.5 inches long. I sent an email of complaint but was ignored.
Still, I was amazed at the thickness that other people experience when they hold my dick. It was enlightening for me. Also, the feel of the silicon (?) rubber was really good.
So, you really big guys, watch out. Order extra liquid rubber material. Do not believe everything that the company says. Guys with 6.5 inches or under should have no trouble casting their entire dick.
I have been thinking about the problem and I think I have a solution. I wish I thought of it when I was casting. If the liquid rubber does not come up to the top of the mold, insert a wooden dowel (about 1" in diameter-depending on your thickness) in the center to displace the rubber and allow it to rise to the edge of the mold. You will have to devise some sort of holding device, but I leave that up to you. Then, when you remove your cast you might want to leave the dowel in place.
Good luck!