If I didn't have the awesome, wonderful, terrific (trying not to exaggerate Ukrainian made fixture I use and was trying to keep the costs down, I would use wood blocks. That is if I had woodworking skills like Tandem Tom. They will look similar to the aluminum blocks Mark sells at Paragon Machine Works. Of course the Paragon blocks would work well too. I would draw my frame design on MDF board and place the wood blocks on the drawn design. It would even be possible to screw the wood blocks laid on their side to the board so they wouldn't move. I would use some kind of metal plate (cookie pan turned over?) to put under the area/joint being brazed. That would catch the melting flux and keep the wood from burning. All you're doing is putting 2 small tacks on each joint to hold their position and then going to some kind of flat surface to fine tune its alignment for the next braze.

Kontact, I wouldn't pin anything unless you have a very accurate fixture or you're setting your frame up on some kind of accurate surface plate. Otherwise you're likely to be pinning the frame slightly out of alignment on your home made holder and your pins will always keep it that way.