I just said I tried hollow slabs. And with the vacuum hopper directly underneath it.That's why you include a hole in the floor that it's in, just chuck a hollow cover on top or something so items can get through but mobs can't. That and put it near your kill hole to lower chance of items escaping.
Hm. That is actually maybe a good idea. Although, hopperhocks are quite expensive and the vacuum hopper would still pull the items. Plus, the vacuum hopper can't collect exp through walls anyways, just as players can't collect exp through walls. I've tried putting it in the corners of the surrounding walls but it for some reason can't collect all of the exp, just some of it.Consider using the Vacuum Hopper for just XP, and use a Botania Hopperhock to collect drops, since it can collect through solid blocks. Note that I'm not playing the pack currently, so I've no idea if you can access them before starting into TC.
Also, as a total aside, does anyone know of any devices that break blocks automatically using a tool I give it, preferably one that doesn't require power or requires very little power? I know the autonomous activator is one, but I was wondering if there were any others.