Been using Powerbox since 1993 and never had a problem combined with Deep Tuttle since 2004 and never had a problem, so I think that Ken's suggestion may be helpful to you, particularly as you say that you've lost bolts even on the Tuttle system.
You might make sure that the bolts are not "bottoming out" in the fin before they are fully tightened in the top of the board. If you remove the brass nut in the PB/TT fin and can see that the fin material is crushed in the area where the fin bolt tightens, this is where the problem may lie.
Other than that, you need to apply more torque when tightening the fin bolt/s.
Hope this helps.