A circle inside of a square is gonna give you a lot less room to grow since you're gonna have to keep everything towards the middle. I'd say individual lazy susans for each plant so you can use the corners of the tent, and just get a watering can with a long spout or a sprayer to reach back thereI'm leaning towards this 24" for $63. If I went with a 46" platform of 3/4 or 1" wood, the 11" of over hang should hopefully not bind up if I have an uneven load on it. So basically I think all that would be needed is the bearing, a 46" pc of wood and maybe a 24-30" pc of wood to set it on. (Due to carpet under the tent.)
Amazon.com: Dailydanny Aluminum Heavy Duty Lazy Susan Rotating Turntable Bearing Swivel Plate Hardware for Dining-Table (24 inch (16mm)): Home & Kitchen