I'd recommend renting from YYZ as the minivans are parked with keys in them on the lot for Emerald Reserve. That way you can select a Chrysler van (and even get your pick of which one). Last year I got one with 40 km on it and Sirius radio.

YHM is a franchise with wonky hours and may not carry stow n' go vans like the corporate locations do.

Finally a Grand Caravan is supposed to be able to fit a 4x8 sheet of plywood on the load floor, so that makes the cargo area 48" wide. So a North American double boxspring is going to be partly on its side laying down. I'd load it in upright and it should fit alright.