How to build canted riser for queen mattress?
"My wife has found that sleeping on an incline helps. I'm trying to test this." To confirm her findings, or that you can sleep on it? --Two cinder-blocks and one brick makes for 20 inches to test it. Confirm this and then proceed. Personally, I'd build it like a deck:

The structure using 2x6's or larger, joist hangers, blocking, and on 16" centers. With 3/4" cabinet grade plywood as the 'deck' and sides, finished with poly. Keep in mind that unless you own a table saw or (even) a very nice circular saw, the angled 'faces' will be hard to cleanly cut. Try to keep the factory edge at the top, where it will show the most. Ripping the angles on the joists isn't going to be fun without one, either.

Kind of like this (not exactly built the way I mentioned), but the pitch makes it angled, it doesn't 'account' for it, like in the picture: