Barrels on wheels, very stable. Even my giant cats didn't tip them.
Banana Gallery - barrels
You can see the barrels through the door. Grow lites on timers. Heat is from tank heaters. In fact, if you have barrels with trees I recommend a tank of heated water with goldfish. You can use the fish water as fertilizer (they are fertilizer factories, omg) and the humidity makes it nice and cozy. Also a heat sink in case of power failure. They keep the room from freezing for a couple of days.