Danno-
How did your "old" rear mast support attach to the trailer? You could rig up something like in the thread on the Inter 17 mast support/raising- an "L" bolted to the trailer rear crossbar that the rear mast support slips over. I used a 2" receiver "box" U-bolted to the back of my trailer and my mast support (2" sq. tubing) slips into the receiver and is pinned there. How far from the rear beam is the rear of your trailer?
Kirt
Taipan 4.9, Prindle 18 and others...