Frames are perfectly fine. You may be cringing from a perspective of trying to streamline everything down to the lowest common denominator but I don't think many people cringe at frames, and the alternative is actually more complicated to pull off.
With no frames, you either allow solid blocks to be picked up, which is a bad idea since players could reposition full health blocks in seconds even under a zombie attack, or you don't allow them to be picked up and you still end up with the placing mistakes, except without frames to palliate them.
I like building frames quickly and cheaply first to give me an idea of the shape and measurements of the base I'm going to be making, and then replacing them (not upgrading them) with the actual flagstone blocks. Could simply make frames be generic and allow you to upgrade them to either wood or flagstone.