G'day Jules.
Very simple mate. Cut a piece off a 90mm poly pipe about 1,1/4" long, slit it and wrap it around your piston and mark where it overlaps, cut the overlap plus 1/4" off, smooth off all edges. Lightly oil the rings and inside of the poly, ensure the rings are in position (no gaps over the pins or fore an aft). Use a 75mm hose clamp to compress the lot putting the gap of the poly at the rear and the screwy bit of the hose clamp at the front. Tighten the clamp till it won't spin on the piston and then back off 1/2 a turn till it just can spin. Lower the barrels on slowly and square. You can see at the gap in the poly once your down past the oil ring. Now undo the clamp fully and pull it out (I move it up over the barrel skirt first, just incase), then the poly can be flexed off the piston up onto the skirt and pulled off. Refer to pic #72.
Cheers