Getting things square is a piece of cake! And you don't need 4 hands, 4 corner clamps, etc. All you need are a couple of tricks of the trade and knowing how to sequence the work.
For starters, visit my website
http://www.railwaybob.com/Modules/ModConstr/ModConstr01.htm
and read how I put my module frames together. From this, you will develop in your own mind how you might want to assemble those pieces of wood. You won't necessarily follow my techniques but, from these methods, you will develop your own ideas as to how you want to do things.
Secondly, decide how thick is your styrofoam deck (assuming you are using styrofoam) and get a couple of pieces of 2"x 4" (actually they're 1½" x 3½") cut to the thickness of your styrofoam (eg I use 1½" styrofoam so I have about 4 pieces of 2"x 4" cut to 1½"x 1½" x 18" long.
Thirdly, don't try to cut the wood yourself. Most building supply stores worth their salt have a saw service using equipment that you couldn't even afford (my chop saw cost me $800 excluding the stand - but that's another story). Get them to cut the wood up to the size you want. It may not be exacto-mundo all the same length but it will be very close and very square.
Lumber - cost doesn't mean anything. It's the straightness and lack of curves or cupping. I've had lots of luck with knotty pine. It's simply a matter of selecting the good pieces that are nice and straight. If the board curves out, it can always be brought back into alignment with some cross bracing in the middle of the module frame.
So, now that you've got your lumber selected and cut, label each piece as to whether it is an end plate, a side piece, a cross member, etc. Also, label each piece, both sides as to whether it is the top and whether it is going on the inside or the outside.
Mark and drill the holes in each piece of wood. Install the T-nuts. You're ready for the next step - assembling the pieces.
Bob M.