You might want to look at the FOLD-A-COVER brand HD folding panels... Locks and looks nice...
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Fold up behind the rear window. You can have a nice tool box under it that is flush with the top rail...
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You can also just open the front section or the rear section...
This can be used with th e fifth wheel hitch setup as well and when not pulling the trailer you can close off the whole top.
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I'm not on the road anymore with my 5th wheel so it has all kinds of safe and dry room for all my stuff when pulling my OFF-ROAD POPUP trailer...
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Just search online for FOLD-A-COVER G4 ELITE... I got mine back in 2008 for around $550... Have moved it to a newer 2010 truck without any problems...
FOLD-A-COVERRoy Ken