Those have a loose jaw, correct? Just cut you a piece of wood the right thickness to slide under the pan when it is level, but thick enough the pan can't drop enough to go off. Lift up the loose jaw, slide that piece of wood under there and after the trap is bedded solid, but before you cover/blend it lift the loose jaw up and slide that piece of wood out.
Of course I'm a tough love jerk, I'd tell him to be careful, he gets pinched a couple times and he'll learn. I've never been caught in a 330, I respect those things, but I've been caught in most other models of traps that I own at some time or other, apparently I don't respect them enough. Most of them don't hurt much, there are only three that I remember hurting very bad, a Bridger 120 across both thumbs (those little buggers are strong, and they hurt!), a 4 coiled TS 85 got me on the side of the jaws where the lever was, took me about 5 minutes to get out of that one, couldn't get either setters or my foot on the lever because my hand was in the way) and the last one never actually caught me, I was working on an Alaska 9 that a moose had bent in the shop and snapping it while holding on to the baseplate from underneath to see if I had it working smooth. About the third time I did that, I had my middle finger curled over the baseplate where the lever touched when it was closed. mashed my finger between the lever and the baseplate, split my nail and I had to walk across the shop to get the setters and get myself loose. Left a blood trail from shop to the house and ended up loosing my nail on that one. But them 450/550s should just teach him a little respect without doing much damage if they pinch him.