Another option is the Harbor Freight special trailer. It's a 5 x 8 collapsible trailer. Make sure you look into that one. I've never had one but many people on here have built with them and they run $279. All you have to do is bolt two pieces of wood or extra channel inside the joint where it's meant to fold.
I don't know where you're located but my local Harbor Freight is advertising this right now. I know they only sell this trailer at certain points of the year but I don't know if it's the same for all regions.
The main mistake that I've made so far with my build was not looking at the weight of the trailer. I seriously didn't even think about it. I can confidently tell you that no matter which bolt together you pick you're still going to be 200 lbs under what I'm stacking.