Sorry for the late reply, I've been away again

Harnass feels very secure to me. You have these two straps that go from the front, through your legs and then you need to tie them to the main harness part. The instructions say you need to sew it but I just tie it and if I feel it's too lose I can just slide the bands a bit forward to make it tighter.
Sounds weird but it sense when you try it

Also, too lose doesn't mean fear of losing the packet. The band around your waist should fit good enough for that. I've done some jumping and such at festivals without even thinking of the packer.
I do wear boxer briefs though, wide boxers might work different.