Not sure how many GBE's are closeish by but................
How hard would it be to find someone else with one and take a few measurments of the boards length width etc and then make a template of the inside of your case and make the board a few MM smaller?
If the case is square, take a foil pattern from the other boat's board
Rudders? what did they have , transom hung kickups?
Again, measurents and templates from another boat
and would the foil be the same as the board?
Just a suggestion