Sounds like your trailer might be rigged for more weight than your gear is throwing at it. Just like an unloaded one ton pickup rides like a** compared to an unloaded half ton, a heavy duty leaf spring on a trailer needs considerable weight to make it ride smooth.
FWIW, our band uses the guitarists Harbor Freight 4x8 cargo trailer that we enclosed with a basic plywood box that is bolted on and fiberglassed. I wouldn't want to ride in the trailer, but I have never noticed anything disturbed aside from my band mates in it...
My gear is pretty simple carry wise. Mixer is in a semi rigid / soft case. It kind of looks like a laptop bag, but has PVC stiffeners in the front and back.
We tend to play a LOT of outdoor gigs, so we keep hand towels in plastic bags to wipe sweat off between songs. (it's hot in Texas!)
So a change or street clothes go into the tote, then the mixer, then the hand towels. Then the spare mixer (XENYX 1202FX) and power supply get wrapped up into the towels before the lid goes on.
Our Powered speakers, combo amps and cabs all ride on 3" thick black foam pads from the craft store. They are in amp / cab covers typically with moving blankets over that, and are cargo strapped to the sides
Instruments are in cases, and are strapped in. Totes are strapped together, and then strapped in.
So I guess you could say we rely on a LOT of padding, and cargo strapping to keep things cushioned and in place. The results so far have worked well for us.
A friend of mine has all his stuff in one of those roll out road case rack things, They strap it to the inside wall of their trailer with a thick foam pad on the bottom. He's been gigging like this on and off for 20 years. No problem.
