Are you trying to get zombies to attack support structures or are you trying to keep them from pulling the rug out from under your base? If it is the latter I would suggest making some new blocks that are specifically for support and giving them a whole lot of durability. You could also take a look at RoyalDeluxes indestructible blocks mod if you really want them zombie proof. https://7daystodie.com/forums/showthread.php?103985-Vanilla-modlet-collection