Originally Posted by pski
No special clamps. Only bar clamps are needed if you have enough to do the full square at one time.. and a flat surface
Of course, "picture frame clamps" x 2 work very well. The issue there is whether they are big enough x and y.
Doing a box with bar clamps takes a couple of people, for me anyway, and the slip factor just isn't worth taking the chance. Mitre clamps on the other hand remove the guess work and the heartache of something possibly going wrong. You can make them yourself if you have a few extra minutes and a little skill under your belt. I don't bother when the tape method works so well.