Has anyone else had the problem with Jet Pack sag? I made my harness following TK409's guide, but what seems to be happening to me is that the front of the harness (on the chest) seems to slide up and the pack slides down. After a few hours, the jet pack is nearly on my butt . Not sure if it's just the straps I used aren't strong enough or what? Anyone know how to fix that or have another harness idea that doesn't do this?