I imported my Uni-go trailer a little over a year ago.
In order to qualify for import, the builder must have some form of certification, unfortunately I forget what it's called. You'll have to phone RIV, give them the year, make and model of the trailer, along with the manufacturer's name and address, and they will check the list and inform you if it can be imported.
When you pick it up, you'll need to get a Manufacturer Certificate of Origin, which you'll need when you register it.
You don't have to take it through the US Customs export form process, so that saves a bit of hassle. In BC, no inspection was required either, so that saved time and another trip to Canadian Tire. Check with your provinces DMV, it may be different where you live.
The bad parts is, you do have to pay the $195 (+ tax) RIV fee.