The soft trailers are very common "Dog Trailers", and would probably work.

The hard cat trailer above was something I assembled.

Bike Friday sells trailer frames.
https://store.bikefriday.com/index.php?cPath=51

They are really a very simple frame.

The cat cage is something I had and is probably 30 years old.

3 holes must be drilled into the bottom of the cage which hold it onto the trailer with wing nuts, and the wing nuts hold the trailer together.

This was made from a Bike Friday cargo trailer that had a crate very similar to the second crate posted by [MENTION=54042]rhenning[/MENTION]. At one time, Burley was a Eugene based co-op, and there likely was some collaboration between them and Bike Friday.

Bike Friday uses a unique air hose coupler which seems to last quite well. Many of their bikes have the bike end welded to the frame, but it looks like they've updated the universal hitch part.

https://store.bikefriday.com/product...ducts_id=20100

The cargo trailers are slightly narrower than their standard suitcase trailers, but could likely be easily modified. Or, it is quite likely the company would be able to cut to your dimensions (be aware of wheel clearances).

They have seen my trailer, and got a kick out of it.

https://m.facebook.com/bikefriday/photos/a.180836545204/10156737821700205/?type=3&locale2=sv_SE



You could probably build something similar out of any kid's trailer or cargo trailer by striping it down. But the frame would be much heavier.