i'd just press my scraps flat, stack them by size (smallest on top), lined up as closely as possible on one side.
cut the first straight edge through the whole stack.
rotate the stack (or the mat if you have a small one) and cut the first perpendicular straight edge.
rotate once more and cut the third straight edge.
after that, cut squares along the whole stack as though you were using strips.
discard the leftover bits from each layer or save them for a time you want smaller squares.