In Canoe gear bags for cartop?

I travel with my canoe on top of the car keel-up. Is there a gear bag that is desiged to be tied into the canoe so I could safely carry some extra gear in it while traveling? Or is there a way I could adapt a regular bag, duffle, etc. for this purpose? It just seems there is a lot of empty usable space inside the canoe when I’m traveling.

Thanks for your suggestions.

Boundry Bags
Seal-Line Boundry bags work particularly well for under the boat travel. The are huge, waterproof, and have plenty of tie down locations.

no problem
We took a family vacation form central NJ to Inlet, NY with a jonboat on roof racks, with a load of stuff between the rack, under the boat.

This was on a Toyota Camry(a great car, execpt for the oil leak).

The only real issue was that I had a flat strap for about 100 miles.

Just use your head, and you’ll be fine.