Skates,
just a question- is that MDF you used for the frame?
I have very bad experiences with that I would like to share.
I had a friend and his girlfriend with a mounted subwoofer in the rear. The sub was mounted very solid with washers and bolts just like you did with the hi-lift.
he was rear ended by an idiot and the sub was ripped out of the MDF.
Smashed through the headrest into the neck of his girlfriend. She was dead instantly.
After a couple of weeks he committed suicide, he wasn't able to handle the guilt of being responsible for her death.
A hi-lift is already very heavy- just have in mind that the weight can increase 10 to 100 times in the case of an accident.