You could use aluminum channels to make a pseudo frame , then cut the outer perimeter and voila, you have a tailgate ( use precision tools so you have a perfect line ) , then use Defender door seal around to water tight as much as possible , giving you a nice finish as well.
P.S. mock the hinges before catting for a nice alignment
1988 110 Tdi
1987 110 2.5 N/A
2006 D3 HSE7