While musing ideas for tank designs, I took a 1.5-gal. clear glass tea-jug w/handle, made sure the drain/pour spout at the bottom was water-tight, rinse-washed it thoroughly with the hottest water I could and added a Tetra Whisper 3i in-tank filter with a sponge over the inlet for coarse mechanical flitration. Since it was air-powered filter, I merely employed a small, battery-powered air-pump--found among fishing supplies in the sporting-goods section of any local superstore--and ran the air-line & hung the pump's clip through the pour/fill hole in the jar's lid.
Now, whenever the hospital-tank was occupied and I needed to quarantine a single fish from the rest... or needed to take one or two to be examined by my local aquatics-veterinarian... I fill the jar with tank-water to just below the filter's spout, place the intended fish inside and add a battery to the air-pump. People would marvel at it as I carried it about, and be amazed at how I made such a "cute" fish carrier w/functional fitration. Even the vet was impressed!
If I were to do this again, I'd look into a jug made of BPA-free plastic--for safety reasons--and see about adding a nano-tank "smart" heater... If I can power it with a reasonably-sized battery.