Here you go:
First pic is the fitment on my trailer: 235/70R16 tyres (what you would find on an old model Discovery) onto standard 5.5x16 Land Rover Defender steel rims. For comparison, the spare is still the original 13" tyre fitted by SBS. Excuse the kelp
Second pic is the footprint you'd get with a 33x12.50R15 tyre - these are mud-terrains for my Defender, I'd get a less aggressive pattern for a trailer tyre.
Mudguards were from Trailer Tek Plastic Mudguards
the ones sold for 16" tyres about three quarters of the way down the page, product code MG126. I just made the brackets out of some angle iron, sprayed them with cold galvanising stuff, and U-bolted them on to the frame. I'm not sure if they would cover a 12.5" wide tyre sufficiently for UK regulations, worth checking that out.
I reckon it would be easy enough to get a fairly standard twin axle trailer specified with the right stud pattern, move the axles apart a bit and change the mudguards to accommodate the bigger tyres, and swap the wheels and tyres for something suitable. It avoids getting into anything heavily customised and correspondingly expensive!
Ben can certainly get the bits, as he supplied the spare drums I just fitted to my SBS trailer
Hope that helps?