Thought l'd throw in my 2 cents.....
You 'can' make them similar to those pictures with a steel frame, then cut a sheet to size and stick it on (its what l did), the problem is that the ute tray is NOT flat!
It curves in side view (drops down at the rear) and on mine also was higher in the middle than either side.
I used 6mm steel rods to help raise the sheet where necessary and give the smooth curve.
It has to be the cheapest way, with around $80 in steel tube, $50 sheet and $50 hinges and sikaflex. Struts can be expensive, and finding suitable ones even harder......
Also best to use something like a fairlane boot latch, which can run both mechanical and electrical release, so no need to external key locks.....
Anyway, check out my album for some bad pics.........
http://datsun1200.com/modules/myalbum/viewcat.php?cid=106or go to my photobucket for some more:
http://s108.photobucket.com/albums/n27/andos1600/