Does it have to be strictly circular? Would a square loop of the same interior area be sufficient?
If the latter is true, you could build it out of something like electrical conduit, or 2x2 lumber, in the form of a square, with diagonal bracing. At each corner you could construct a sort of U-shaped bracket, with the open end of the "U" facing outwards. This would serve as a retainer for the loops of the coil.
Moveable? Are you hoping to move it "intact", or break it down somehow, or ???
How big is your trailer or other vehicle?
If the amp is only rated for speakers with a nominal impedance of 8 ohms, then you probably shouldn't go below 8 (or perhaps 6) ohms DC resistance.
You're talking about roughly 60 feet of wire per turn, or about a watt and a half per foot.
If you want a single-turn loop, 8 ohms, it would work out to roughly1/8 ohm per foot, which means something like 31-gauge wire. That's thin and fragile. It might dissipate a watt and a half per foot without burning up but I don't think I'd bet on it surviving.
If you went with something like 20-gauge wire (about 10 milliohms per foot) you'd need 800 feet of it, or about 13 turns around the loop to make up 8 ohms.24-gauge would be 25 milliohms per foot, requiring around 320 feet for 8 ohms. Roughly 5 turns. That's probably in the right ballpark. Since heat dissipation per foot would be much less (300 milliwatts per linear foot of wire) it should be safe against burn-up. 24-gauge stranded hookup wire is easy to come by, and flexible enough not to break too easily.