Hi Philip,
Welcome to the forum... I'm not completely understanding your question... do you want to have a frame fabricated or do you want to buy something like the HF trailer....
Fabricating a trailer... check out my thoughts here...
http://www.mikenchell.com/weekender/chassis.html
To buy a red trailer, check out the Red trailers...
http://www.redtrailers.com/TrailersFolding.asp
To get to 5x10 which is what I think your question was, you are going to have to fabricate...
Mike...
P.S. What's stopping you from using a design that lets you build a 5x10 trailer on a 5x8 frame?
P.S.S. Most cub/modernistic designs use a reduced size frame...