If the cap was an exact fit, then the roof mount flange should be available from the same manufacturer.
How the part is installed may be the issue, the mount flange may be under the fiberglass roof , and require fridge removal to change it out.
Do you have a fiberglass roof?
Any chance you could post a picture?
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.