Alfred, a viable alternative is a set of aftermarket louvers covering either the opening, or the whole depressed area of the rear deck. It transforms the car's silhouette to something similar to a Lambo Miura, and does prevent rainwater from getting in. These attachments were popular in the '70s & '80s and although rare now, a set could be fabricated locally from vintage photos. Or, Kirk Evans' "Amerisport" built/builds a fiberglas rear screen with a 2" or more raised center section just for the purpose you stated. There ARE ways....
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.