I made my own square out of MDF. Simply use two sets of dogs (I really like Parf dogs from Lee Valley). Use one set to establish a reference edge. Then use a second set to square a track saw guide rail (or any other parallel guide if using a circular saw) at right angles. This will yield a very precise 90 degree corner to be established provided that the set up is secured well for the cut. I think my square is about 460mm on a side and cost about $10. It was very cost effective and quick to make, and if I ever damage it, its not a big deal.
I use this square to set up the MFT and also for occasional verification checks and it has yielded perfectly square cuts that have produced many parallel and square cabinet parts and assemblies. Its also easy to verify using the square check method described earlier. I wanted to buy one of the pretty, but expensive squares, but opted to spend the dollars for a few sets of dogs which for my purposes yield more value.