You can get some tie down anchors and use epoxy to attach them to your shell.

And you can get a grommet kit to add grommets to your tarp for a custom fit.

I've been thinking about making one of these too so I can camp out of the back of my little beater Chevy Tracker. It's not long enough to sleep in so I'd have to make a bench to extend it for that, but that's pretty easy to rig up.
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