I was surprised by how little options there are on the market, you'd probably have to fabricate it yourself. I've seen a few hobbyists in facebook groups making custom boat trailers, you might start there.
Plenty of parts would be available off the shelf like axles and miniature metal grating, but there's doesn't seem to be much in the way of 1/10th hobby grade (or even toy grade) onroad-style trucks & trailers. There's some very nice 1/14th stuff, but basically just the TRX-6 in the 1/10th hauler category. It's tempting and looks fun, but I wonder how often I'd actually be bashing it with a car on the back. It could be a fun crawler challenge to take an onroad car all the way around a dirt course on the back of a hauler.
There's one or two models of ball-hitch flatbed trailers, which tend to be a little wide for onroad cars- looks like they were designed around the SCX10. Their ramps would also be too steep for onroads.