I wrote a couple of instructional posts on doing gallery wraps a while back, here:

How to Make a Canvas Gallery Wrap

and here:

Adding Borders to a Gallery Wrap

I use Dick Blick heavy duty stretchers, but the medium variety are almost as good, just not as deep. They slot together quickly without tools, although a square and a rubber mallet come in handy.

Tip: to make sure that the stretchers are square all the way around, use a tape measure and measure diagonally from one corner to the opposite corner. Then measure the other two opposite corners. The figures should match. If they don’t, then tap the frame on the side that needs adjusting and remeasure. You should be able to get within an 1/8″ or better tolerance with good stretcher strips. Generally, if you tap the corners and square it up first with a small try square, you will be close. The tape measure trick works best as a final check.