The three plywood plates I designed in Inkscape and cut on my CNC router. The reason I went for a threaded bar design instead of a base is that the central plate can be moved within the rails and can therefore be used for very short rocket air frames or can be made to hold the body tube securely whilst avoiding a body tube anomaly such as a camera housing etc. Using threaded bars also gave me maximum adjust-ability and meant that I could assemble the jig with utmost care for parallelism and making everything square.
Above is a shot showing the adjustable rails that can slide left to right and then be tightened into position. The position is accurately set by inserting a fin into the V section to centre it and also placing a spare fin or a spare piece of fin material at the far end of the jig into a matching central V slot. Then the jig rails can be tightened and after an alignment check the fin glued. Then the tube can be rotated and realigned without resetting the jig to attach the next fin. The fin setup can be seen below with a fin and some fin stock at each end of the adjustable rails.