Use some filter media or filter sponge that's been in your main tank for awhile -- put it in a powerfilter or cannister filter to establish a biofilter in the QT. Use PVC pipe/fittings to provide your QT fish with shelter and hiding places. Don't use LR. If you have to medicate, it'll ruin the LR. Keep an eye on ammonia/nitrites/nitrates. Do water changes frequently, and step them up if ammonia is elevated.
You should not have to leave your QT set up for months on end with nothing in it. By keeping a little filter media in your main tank, you should be able to set up your QT very quickly when needed
HTH,
Mariner