You use a pad or some other insulation under your legs. There's a reason why 3/4 underquilts are so popular; with full length quilts there is sometimes an air gap under your knees, which allows convection to cool your legs and sometimes your butt.
There's also a reason why full length pads are so difficult to position correctly. They curve in two directions - or at least your body makes the hammock do that, and a flat pad doesn't want to comply. The section under your feet and legs, however, curves mostly side-to-side so it's easier to keep your legs in the nice tube formed by a short pad.
That's not the whole story, but there's some truth to it, and it's probably cheaper to use a 3/4 UQ and a pad than a full UQ. All the usual disclaimers apply (HYOH, YMMV, etc.)
I use insulated hammocks, which (IMHO) don't have some of the drawbacks of full length underquilts, but which have their own set of complexities.
There are others far more experienced with this question than I. Please chime in to correct my errors.