Clamping corners at right angles is real "iffy" without some type of jig. There are plenty of jigs out there for sale. Rockler, for example, sells several versions of their Clamp-It jig. Several companies make and market these jigs, and it's amazing how inexpensive they are. I contacted one of our local Jacksonville machine shops (A & K Machine) to see what they could do...
So, I chose to make my own. I saw an improvement update to a Right-Angle Clamp Jig in my most recent Wood Magazine (Issue 174 -- Dec, 06/Jan, 07). Oddly, I seemed to completely miss the original article, but the update sure caught my eye. Anyway, the original clamp jig had a 2" hole in the center, while the "improvement" that someone sent in had a triangle. The triangle facilitated more clamps. I liked it, so I made some. Once a decent pattern is made, you can crank these things out in about 20 minutes or so. Also, it's easy to adjust and make them in different sizes to use on different size jobs.
You don't need much in the way of materials for this jig. A decent 12" square piece of " plywood will yield two of these clamp jigs. Go ahead and cut at least two of these 12" pieces... you'll quickly realize that two of these jigs aren't enough, but four of them will get you through most projects. And, if you decide you need more, you're just one square foot from another two jigs.
For jigs I use often (like this one), I like to make patterns for them. I've found that using "production" jigs to pattern new copies often delivers less than optimum results because they're battered and bruised by the time you attempt to copy them, but patterns are pristine. And, I can rathole a pattern for many, many years. I usually make patterns out of MDF because it's cheap, and if I screw up, I don't feel so bad about throwing it away and starting over.