I hesitate to shill for my own trailer, but it is for sale, and would make a great GT40 race trailer.
It's a 20' ATC all-aluminum trailer. You can see it at Racing Junk at
Race Ads | For Sale: ATC Aluminum Race Trailer- 8.5 X 20
It doesn't have a side escape door, but the safer way to get a wide car like the GT40 into any enclosed trailer is to use a small electric winch. Driving a relatively wide car into a trailer is pretty risky, IMO.
PM for more details if you want them.
I'm selling the race operation to buy a car from RCR...