I have taken this kid's trailer (Chariot model no longer in production) apart to turn it into a cargo trailer, and possibly a micro camper too.

I sometimes haul heavy stuff (e.g. compost, small furniture) so the floor needs to be sturdy enough to withstand 50-60 kg of weight.
I am getting stuck at floor options: Marine plywood, aluminium, PEHD, Coroplast, foam tiles, linoleum?

What did you use? Or what do you recommend?

Chariot Corsaire

I’ve had good luck with shelving:

The post your trailer thread.

For high point loads, I have a sheet of 1/4” plywood that I use to distribute the weight.

3/8 plywood on top of the frame

I’d look for 1/4 HDPE it’s light strong and of course waterproof and extremely easy to work with. drill a hole and zip tie it to the frame. The other option may be aluminum which you could still cut with a circular saw but would have to do more grinding on the edges for it to be less abrasive to the zip ties. Screws will only as weight and the vibrations may work them out of the frame.

I’m thinking the plywood may hold moisture and add weight.

What a great idea. I have some big 50 x 57 cm wire baskets/shelves in the attic that I was saving for something. Appearantly it was this. I'll give it a try.

I wondered if it was maybe too brittle; likely to break with heavy loads.

You will need some section to support the span. The nice thing about the shelves I used is that they gave me about 3 cm of section at each end and in the middle.

Aluminum angle might be an option to add stiffness.

should handle anything you want to pull. Use the frame as support, so the platform is on top of the frame. Pretty confident you won’t be able to break it unless you are using it at sub zero temps. I occasionally work with it and it is stronger than 3/8 plywood and think it would be more durable than 1/2” plywood. I’ve tried to bend it with a heat gun to bend pieces to make diy cargo cages and I gave up, was able to bend it but not very well and none broke under my manipulation.