You could just relay on glue to keep the joinery together however, inserting splines offers excellent joint reinforcement and will allow you to build larger boxes and frames without the worry that the joints might come apart a few years down the road.
In fact, this jig is definitely one that I will use on a regular basis. And want to know the best place to build one. Thats right, I made it at TechShop in Round Rock (www.TechShop.ws).
Step 1: Materials Used and Cut to Size
The measurements can vary, but these are the ones that I used. Keep in mind that your fence guide will need to differ to fit your fence on your tablesaw.
Cut two pieces of plywood for the cradle using a tablesaw
6" wide x 10" long.
Then cut a 45 degree angle on one side of each piece using a miter saw.
Cut the runner with a tablesaw and miter saw
8" wide x 10" long
Cut the fence guide top with the tablesaw
2 1/2" wide x 4" long (put wood on top of the fence and trace the width of the fence)
Cut the fence guide handle with a tablesaw
6" wide x 6" long