That cover.... I nicknamed it the evil cover. It started on 2004 models I think. The screws are so tiny, like a no 2 maybe a no 4 screw. phillips head no less and it is buried deep inside a blind hole that gets filled with dirt from towing.
Odds are high they had to service one of the self resetting breakers inside and they broke the cover getting it off as they did not figure out the screws.
I converted the screws to no. 8 stainless and went in and stopped at the end of the cover rather then 1" inside the cover. This is what you are looking for.
Official name for the box, h'mm don't know. Maybe "header junction box", or "7 wire plug junction box"
Unless you can find the exact make and model of this really thin brittle plastic box, I do not think a store bought cover will fit. This really is not a NEMA style waterproof junction box. I added 3M 33 tape around the cover to help keep some of the water out.
The 1/4" thick Lexan suggestion is a good one. You can spray paint it black if you do not want to see the things inside. You can make a gasket too if wanted.
Hope this helps
John
__________________
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
Google Custom Search For Sunline Owners Club