Everything in our studio has been custom built. There are no pre-fab pieces. We even modified the fire doors. These windows were a project my dad and I worked on while he was out on vacation.
The general idea came from a diagram in the book "Home Recording Studio: Build it like the Pros" by Rod Gervais. According to him, the best window construction has an inward slope as to cut down on glare, saving eye fatigue and making it look awesome. It's purely aesthetic.
We built 4 windows, 2 per wall. We used 24" x 36" pieces of 5/8" and 3/4" glass, 2 of each. The frame was made out of 5/4 cedar board, 1x4 pine and 1x1 square trim boards (I can't recall what type of wood, probably fir). To seal in the glass, we used 1 1/4" closed cell adhesive-back foam, with Green Glue Acoustical Sealant.
We had to cut it to size for the stops on either side, but the top and bottom were set in so the foam curved up like a U.
The top and bottom were put together the same, a piece of 1x1 on the front and 1x4 on the back, the bottom installed first.
We then cut the 1x1 stops at a 5° angle on either side, cut the foam to fit, attached them to the frame, and slid in the glass.
A bead of sealant was added before the other stop, then we dropped on the lid. Once installed in the wall, they create a V shape.