you could use a jig, I have seen people make these out of lego pieces stacked up. I build my frames on a nice piece of granite.
I glue the cross members to one side of the frame first, then glue the other side rail on after it sets up for a while.
I always use good old fashioned tube glue so I can make adjustments. it also gives a strong bond.
here is a 359 frame I did for my current project and stretched it out a little. hope this helps.