I got the second model ready, and the results are starting to look quite good. I used two different tube sizes for the main frame, and one of them was of course the same 25x25 tube used for the locost frame. The front beam is made of stronger tubing because I didn't have the nerve to make it from anything less. I'm a bit disappointed at the added complexity of the frame, and the final model weighed 230kg.
The picture above shows a situation when two locosts are resting on the trailer. The first model looked quite scary when the displacement was over 50mm at worst, now it's less than 1mm.