The Townshend Pods are about a hundred bucks each from England ($150 in the U.S.), and you need nothing else besides a nice stiff, non-resonant shelf between them and your table. You definitely want to get rid of the MDF---it’s nowhere near stiff enough (it’s ground up sawdust and glue, after all!). I found marble to be too "ringy", slate might be better. Two pieces of 3/4" Baltic Birch ply with a later of constrained layer damping between them makes a great shelf. Or, if you have the dough, the Symposium shelves, which have constrained layer damping themselves, are great. You probably know Symposium has a special model shelf for turntables, which adds their own springs for vertical isolation. I don’t know if they are as effective as the springs in Townshend’s Pods, but I’m sure they’re good.
If you want to stay with the inner tube, it plus roller bearings provides good isolation for the money. The inner tube for vertical, roller bearings for lateral/horizontal.
Ingress Engineering in Canada is making a few different models of roller bearings, the cheapest being about the same quality and effectiveness as the Symposium Jr's but at a much lower price, the most expensive being better than the Series 2+ (the diameter of the "bowl" is much larger, for one thing), for less money. Great products!