Welcome to ribnet!
There is a consensus that says the Indespension Jetski trailers are fine for SR4s - I had one under mine. Last time I looked they were £3-i4sh hundred quid.
The one thing that is certain - any used boat trailer is likely to have been swimming, so the chances of finding a good condition second hand are slim, but not impossible.
How heavy is your humber, and what are you towing it with? Strictly I could go unbraked with mine, but I decided to "future proof" against lighter cars. Just as well, as the new version of what I am currently driving is about 200KG lighter!
I throw that thought in because unbraked there is an argument for just going new, as there is a high chance you may need to spend almost the cost of a new one to make it roadworthy.... That's what happened to me, hence my new trailer!