Welcome to iRV2. Your best bet would be to find a local custom trailer frame builder and see what he can work out for you.
There are also a couple of vintage trailer groups on yahoo that have some Spartanette owners on the group. You might try this one,,,,Vintage_Trailers_USA_Network : .....2,040 members strong and climbing
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot