My quarantine tank (QT) is a simple box of water. Made of acrylic, it holds about 12g of water. I use a small 50w heater, a LED light, and a Vortech MP10 for flow. Some dedicated live rock and some PVC fittings complete the system. Stick a glass thermometer in the tank to verify your target temperature is consistent.
PVC fittings gives the fish a place to retreat to if aggression takes place. Daily water changes keep the water parameters in check, taking water from my reef to refill the QT, and then adding new saltwater to my reef.