Buy a stripped lower.  Have it transferred to you as if it were a pistol/pistol frame.  Built it as a pistol.  Convert back and forth to (legal) rifle config as you like.

The law says if it is built as a pistol first, it is a pistol.  Pistols can be converted to rifles and back again (but each config must be legal, so the long barrel needs to go on first, and come off last, or you'll built an SBR), but rifles (guns that started off with a stock) cannot be made into pistols.

-Troy