Same for us, we can only rescue. Except, when I get my Caucasian Shepherd, it's nearly impossible to find one that is tame (by tame I mean like the tigers at the zoo who don't kill you when you go in their pen) and in need of a home.
I'm sorry for your situation, but it seems like a good home for your new gentlemen.
The way mine works, there is space between it and the vehicle. If it looks like this:
< VAN
The a bag could be secured in the open wedge between the van and it, and the other portion could be secured inside the bike holders. Here is the inside of my boattail.
Definitely enough space for a modest haul. A large duffle bag and two medium to smalls should be easy to fit in. A few cable ties combined with the lack of turbulence from the shield would make it pretty secure. I wouldn't leave it out over night, though. Will bring attention and people would have an easy time pulling your stuff out- especially if you design it for ease of access.