Another option is to purchase a second
HOB filter. Run it on your regular tank to keep it established. For a q-tank you can then just use a rubber-maid container with the spare filter and an air stone. No need for gravel in a q-tank.
This option also allows you to be more flexible on tank size. A 5 gallon tank might be ok if you plan to keep very small fish and only have to quarantine 1 or 2 at a time. But if you have larger fish or ever have to quarantine several fish the 5 gallon would be very cramped.