I can't think of a single one. The only one I know that does that at all is Larue (10moa) and I don't believe they sell their OBR uppers stripped anyhow. Most AR15 receivers require a cantilever mount as it is when attaching a scope, and those are more than plentiful in 20moa fashion. Additionally, if you have a canted base and run a 1x optic on it, you can run into problems that way with the dot/reticle not being centered in the optic throwing potential distortion and multi-plane viewing issues into play. Finally, two different planes on a full length top rail, if you go that way with the forearm, would cause further issues or just plain look silly.
I recommend sticking with a standard flattop upper.