Their own toppers are probably the only toppers they'd ever recommend using. If you don't want to use Kanger however, give Aspire or Innokin a try. A lot of people like the Aspire Nautilus. Less popular is the Innokin iClear X, which is Innokin's glass option. I haven't used that one yet, but I did like the iClear 30b which is basically the plastic version. I moved on from clearos pretty quickly though, so I don't know anything about the longevity of any of them. Aside from rebuilables, I use cartotanks.