I personally just run a bare bottom 40g Breeder as my QT. I dont' keep it running 24/7. I buy a few packs of sponge filters and let them sit in the sump of my DT. When I need to QT, I basically do a water change on the DT and use the old DT water into my QT and use one of the sponge filters to run on the HOB
in the quarantine tank.
If you haven't already, check out cupramine (or some other copper medication) as well as prazipro. Both medications are pretty good as covering your basics in the QT.