Your 33s will almost certainly be too big for their slings, I had 28" rollers and even then there wasn't much slack left. I threw heavy chains around both axels (watch your brake lines) and secured with come-alongs.
I'll second this idea. Head over to Harbor Freight/Tractor Supply and buy a couple chains and some load binders. Use the chains instead of the straps on the trailer. That way you can place the Cruiser on the trailer for best weight distribution not because that's where the straps are.
I've moved Cruisers with both a dolly and a trailer. I prefer the trailer because it has brakes while the rental dollies do not.
Good luck!
Nick