If you want my opinion having never built a glass bow then .... It's feasible but it's not gonna be the smoothest road.
My advice is build a kennyM R/D longbow. The plans are out there, and the stack information is also published. Don't try and use a premade riser block that is meant for a hill style longbow. It's going to look cobbled together. Also if you make you're own design 2 things are likely to happen.
1. It's likely it won't perform the way you want.
2. It's guaranteed than no one will be able to tell you the stack to use. It will all be guesses, and if you ask 10 people you will get 8 answers.
Build the KennyM bow. It's considered by many to be a very good RD design, the stack info is published. Kenny M can also hook you up with the glass and lams you'll need.
Regarding the riser ... the fades are tricky, but the whole idea of building your own bow is to learn. Just take it slow and you will be fine. If you mess up, just get another block of wood and try again. No sweat.
And yes the peg and inner tube forms work fine. I would definately recommend using a pressure strip also though just to help give even pressure.