On anything less than 30" on the long side I'll use commercial 1x3 [nominal] stretcher bars. I buy a stock of them them from various places on-line, wherever the best deal that day might be. If I need a size I happen to be out of I stop by the neighborhood Hobby Lobby and get a single set of four. The bars purchased on the cheap on-line tend to have very small knots and the odd bark irregularity but they're uniformly straight and sound. The ones from Hobby Lobby are always clear wood but thet cost a bit more. Just pop them together, square them with a framing square, and drop two or three staples across each corner joint. No glue, no fuss. Takes maybe a minute. None have ever come apart and no one has ever complained about any irregularities, mostly because 75-80% of the entire bar is covered with canvas.
On larger sizes I use finger joint and primed brick mold. It's the perfect size and shape. Available at any home improvement center or lumber yard. Cut in the miter saw, a drop or two of Tight Bond and the stapes up the joint on both sides. Takes a couple of minutes longer and costs a bit more, but I charge accordingly. Gives a larger more substantial looking finished product.
During the entire process the thing that takes the most time is having to reload staples into my air stapler.
