I find the following technique allows me to trap without much effort. For this example I'll assume a starboard tack downwind under spi uni (solo).

Once on the proper starboard heading (assuming you just jibed or rounded "A"), I hook up with my trapeze. I situate myself near the rear beam with my butt hanging over the hull--ie I'm suspended by the trap' line. Right leg is extended with foot against the (starboard) stay plate. Left leg is collapsed with my knee near my chest, foot on the hull.

From this position I can re-position my weight forward or aft.... or .... extend my left leg and voila' I'm full trapped.