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So you want a set of pieces that can make up x × y rectangles without any of them needing to be flipped over?
I'm 99% sure that is possible based on the following "proof": For example you could mirror every grid vertically, so that the first set becomes a 6×20 grid. For the left hand side of the 5×24 grid you select all the pieces with the correct handedness leaving a mirror set for the right hand side.
The problem with that is you get duplicate pieces, is that something you want to avoid?
Can you explain that again?