New version 2 here: http://www.thingiverse.com/thing:2210380
I wanted a low boy trailer ever since I got into RC Semis. They are pretty expensive with high shipping charges for intl freight. So I made one.
The goal was to adapt the Tamiya flatbed trailer (https://www.tamiyausa.com/items/radio-control-kits-30/1-14-scale-r-c-tractor-trailer-35100/flatbed-semi-trailer-56306) so hardware and the parts that didn't need to be re-created were easily obtainable. This trailer is available from most retailers in the $170 USD range. For the most part, if you buy a trailer kit and print all these pieces, you'll have everything you need. There may be a few extra fasteners required as I didn't track what I used in addition to the kit as closely as I should have. If you're a bit of a maker or an RC nerd, you should have some spares lying around anyway and will probably get you by.
The rear frame rails are the same on both sides, so mirror that part in your slicer to get the other side.
In addition to that, I used some spare center supports that I had leftover from other kits. If you don't have any of those, you can print a few from here: http://www.thingiverse.com/thing:1936011/#files. Its called "support.stl." You may also be able to use the crossbeam from here: http://www.thingiverse.com/thing:1903313. Thanks jan1cato!
There is still a bit of flex in the bed and at the rear in the rear adapter. The adpater flex is probably unavoidable with PLA (perhaps some carbon filament would stiffen this up). I may add a few fasteners to the side rails and the bottom aluminum sheet to help combat the flex in the bed. It is still pretty strong. Unless you're loading lead or gold bullion you'll be fine. I'll probably pickup a Bruder bulldozer or other heavy equipment model for a load for this trailer. The center bed is a scale 32' long, which may be longer than most lowboy trailers. I didn't want to require any metal cutting or fab for this project, so I just left it as is. If you wanted to shorten the center section it would be pretty straightforward and wouldn't affect my parts at all.
There are some custom supports under the rear adapter that break away easily.
While I printed this at 50% infill, you can probably get away with much less.