A little known trick you can do with the factory belts that the head HPDE instructor showed me at the NASA event last November is this...

1. With your right hand, pull your shoulder belt up as tight as you can get it near the lap buckle and hold it near your body.

2. With your left hand, pull the upper part of the shoulder belt out from your body and let it snap back into the retractor.

This should lock the belt. You may have to do it a few times if it's suborn.

Last step to really make sure you are 'tucked in'. Move the seat forward to tighten the belt further.

Cheers!