Being somewhat minimalist, and having, not just a bass rig, but a PA, guitar stuff, drums etc to shift, we make up 'dolly plates' from ply and a set of heavy-duty castors...
Optionally, cut a hand-grip into one end, to ease carrying it empty, and/or hole for a knotted rope for hauling it along. Cheap, cheerful, pretty well unbreakable and polyvalent. Not so good when stairs are involved, I'll grant, but handles Leslie cabs or pianos, too.