I cut 11 Ga. aluminum plate to closely fit the shape of the rear engine tin. Then I used a '67-'74 Bug engine seal to close the gap. The aluminum is in three pieces to help with installation/removal. It overlaps the bottom of the 2" x 2" square tubing frame and is screwed into the frame with self-tapping screws. It has worked very well and is tight. https://www.jbugs.com/product/111-705D.html
James