I suspect you could apply duct tape to the inner surface of the door contact points, set the door in place and then apply the fiberglas between cabin frame and door. The fiberglas would stick to the cabin frame but not the duct taped door surface and when cured would match the inner contour of the door.
There really are no contact points on the doors/ frame other than the rubber seal. I also assumed you were going to install the McMaster seals.
The technique I described is really meant for the final shaping of the gap between door and frame. You'll also need to have your windows installed for this or else as I pointed out, you'll spend a lot of time perfecting a fit that will have to be redone again.
Applying any kind of tape to either the door or the frame will interfere with the parting technique and result in a jagged line.
Once the outer and inner shells are joined, did you trim the door perimeter back to the scribe line, which I presume will allow the door to then fit into the door frame, and then install the hinges and door handle mechanism? Or did you install the hinges and door handle mechanisms before trimming the door perimeter to fit within the door frame? Without the instructions in hand, I assume the first scenario is the process to follow.
You'll want a fit with some room to trim a little more later.
To make any kind of decent fit you must install the doors with the hinges or else your fitting effort will be in vain.
I also installed the closing mechanism at this time as well as the guide blocks
so as to have the door in exactly the same place for final trimming and finishing
One more thing: I installed Nut Plates on the bottom of the door side hinges so I would be able to quickly remove and re install the door during fitting.
I must have done this 10 times or more to get the fit I wanted.
As always, everyone has their favorite way to install the door and certainly there is more than one way to to this.