You can do a parabolic curve by making the radius to the center of rotation bigger than the part and just shaving what needs to be cut off. It's a little tricky but you'd be able to do pretty much anything with a good indexing table, a test dial indicator, and some trigonometry.
Since this is just for weight reduction and to float under the handguards, there's no need to go full retard. The whole business with math, indication, and indexing table would be required if the part profile was critical for, say, a moving mechanism. However, I can use the belt sander since it's easier, much faster, and more than accurate enough with a steady hand. Even when looking at it, it's even and has a nice shape.
Final part thickness is a little bit over 0.125" at the thinnest, over twice the thickness of the original part. Clears the handguards by at least 0.025" in every direction, and the weight difference isn't really noticeable. It's actually lighter now that I've swapped out the collapsible stock, the one I had on it before this had this heavy rubber pad.
Range report probably this weekend.