Just my opinion, but you need to go back to your posts on trailers and read some of the excellent replies you got for upgrading the (I believe it was) folding Harbor Freight trailer with some wood based extensions. Trust me on this: you want to hang as little weight as possible off the back end of a Mustang, which means the tongue weight of a trailer, not some device that hangs a whole kart off the back of any year Mustang, along with whatever spares and tools you put out there with the kart or in the trunk. I have experience with that - the current Mustang is a 2014, and it's the 3rd I've had. I know from your questions that, at least for now, you can't afford a truck, and that's just a minor annoyance, not a big problem. Just don't go hanging any amount of weight you can avoid off the back of any year Mustang, get a trailer hitch and the cheapest trailer that you can modify with some lumber and go from there. Too much searching for cheap solutions, other than the best you can do on a trailer, will just result in $$$ and handling problems with your Mustang.
My 2cents; take it or leave it, but remember that it is based on experience with Mustangs that began in 1967. It took me a lot of years before I could afford a 3/4 ton crew cab diesel pickup (which is now 14 years old and seems like it will run another 14) AND a Mustang, so hang in there and be patient. It will all come together eventually, and you will figure out how to make do with what you have until then.
