I should think anything clear or sort of clear that you can cut to size would work. Clear so you can see the block thru it to position it properly. Now it all depends on the size the block is supposed to square up to be.
The milk jug idea would work just fine if the block squares up smaller than the size of a milk jug. Put on your thinking hat and look around your house. What is clear-ish that you can cut into a square up ruler? Some picture frames might have glass alread in the correct size. Some cheaper frames have flexible plastic instead of glass.
Also remember a pefect square will measure the same on both diagonals. Measure corner to corner on your blocks with a regular ruler and square up from there.
Keep thinking. An idea will present itsself that will work for you.