<< Is there anything I can do to make it stay put a bit better, but still be removable every now and then to add new music etc?>>

Maybe you could remove a few links from the handle's springs? While I always pull my empeg/amp before heading to the track (Willow Springs 2.5 mile "Big Track"), I have never had a problem with my empeg on the street. I drive w. 345 Pilot Cups and 770 HP at the rear, and have never launched my empeg when it was fully seated. My sled is slightly raked facing up, so maybe that has helped. I would modify or replace the springs.

Sean