I took a piece of 3/4" good grade plywood and coated the exterior side really well with 2-3 coats of thin set epoxy which cured a waterproof exterior shell on the sheet. Placed a piece of Fiberglass Reinforced Plastic (FRP) on the exterior of the door. Then made my sealing flanges for the door out of 4" wide fiberglass cloth, that after cure, would be mounted to the outward edge of my door. It worked fairly well. Not the 'most professional looking and certainly not the most aesthetically pleasing' one you have ever seen but it does the job for now.
Thin fiberboard strips cut to appropriate size and wrapped in wax paper to allow me to drape wet fiberglass cloth over the ledges. The idea was that the wax paper would prevent the polyester resin from bonding to the fiberboard and the 1/4" variation in height would create an offset in the cloth thereby creating a 1/4" area to accept weatherstripping so door would shut tight and seal out rain.
Door installed and interior insulated.