The small kart trailers sold in Europe and Australia are really slick. I sent some photos to some trailer shop to have something built up and nearly had my heart stop when I was given price as they were on average 10k! If you change any standard dimension the price goes thru the roof. The really neat thing about the trailer overseas is they build base OVER the wheel so you end up with narrow footprint that still fits kart.
I went with second hand aluminum trailer and spent some time refurbishing the trailer. The cost of aluminum is crazy high and I also decided to use lumber/plywood.