Years ago, I tried to run with this idea. In theory, it's a good idea, but the execution is difficult. We had an extra t-bag
(West Marine Link)
from our boat and I tried to make it work. First I just used the upper tiedown and the rear oh-shit handles. It sagged so far down that I couldn't see out of the rear windown. Then I tried some v-clips that went between the ceiling and roof, but it just wasn't strong enough to hold it and ended up pulling some of the ceiling down which was a PITA to repair. You could try using velcro maybe, but again, the ceiling may be pulled down unless your only carrying feathers up there.
Frankly, it was much easier to build a shelf for the rear cargo area. There are instructions on this site if you look. With that back there, I always have plenty of room for my every day carry, and everything else. An added bonus for the shelf is privacy, no one can even see my gun cases, groceries, or luggage.