It's a matter of time and attention. The longer you can make it take and the more attention gets drawn while doing it the safer it will be.

Upon looking at the product in question, two things come to mind.

First things first, the clamps. A plastic molded thumb turn is great and all, but super easy to undo if you can access it. Fortunately this is super easy to fix: Jam nuts. Get two nuts that match the threaded rod. Run them up to the bottom of the thumb turn. Now get a pair of wrenches and tighten those nuts as hard you can against each other. No risk of damaging the thumb turn and no way to undo the thumb turn and unclamp the cover without using a wrench to un-jam the nuts. Best part is that the nuts are super cheap, this retrofit will easily cost you all of $4 and make it more annoying to deal with the cover even after cutting through it.

But the main problem is that the fabric will not, cannot, stop or even meaningfully delay forced entry. Any yahoo with a pocketknife can just cut a slit and get right in. So my first initial thought was ... chicken wire. Or better yet, welded wire fencing. For $50 you can pick up 50 ft x 40 inch rabbit guard wire fencing from Tractor Supply, which should be plenty for what I have in mind. Go to a big box home improvement store or a farm store and see what's available.

You'll have to figure out some way of securing the fencing to the framework of the cover. I'm thinking a small diameter drill bit and using wire ties. Drill a hole straight through the support bar, wire through the holes, twist around the fencing, and there ya go. Since the fencing is flexible you should still be able to open up the back like you're supposed to.

With the wire fencing on the underside even if someone slashed the cover they could probably still bend the wire to get their hand through (depending on the size of the grid), but they're not pulling anything out of the bed through it. If they had enough time and privacy they could cut slits, bend wire out of the way, and unhook the cover from the bed... unless there was something preventing them from unscrewing the hooks, like a jam nut.

Sure, if someone had time and motivation they could get through the wire fencing pretty easily, but the only thing that can really stop a determined thief is being there and stopping them yourself.