A heat lamp pointed at the tubs can heat them. I've done that in a pinch several times before.
Also keeping them all in a closet with a lamp on a tstat could work. The lamp should keep the temps up in a small closet. Plenty of people have done that as well.
I wouldn't be opposed to putting them in a closet but I have a weird closet that has built in shelves.. RIP. How large of a heat lamp and where do I find one large enough?