Two topics:
First Fenders:
I am still planning. I thought about putting on cheap metal fenders and then putting my custom made wood fenders over them, attached to the side of the trailer. This method would allow you to put on the cheap fenders and camp now, and install custom fenders later. I’ll undercoat my custom fenders with auto undercoating sealer, but the metal fenders should take all the wear and tear from road debris. I wondered if there would be room to change a tire myself. I can check clearance before building the fenders, and if there looks like there won’t be clearance, make them so that the outside skin can be removed. This will work for wooden fenders, and possibly for aluminum if you leave the trim unfastened on top, or just take out the screws if you have to remove the side. In the first diagram, I failed to show the screws holding the frame to the trailer side. Use your imagination...
This diagram does not include the metal fenders underneath. This method hides the bolts used to hold the outside skin in place.
Second, Aerosol foam. Yes, it will expand WITH POWER. I put in new vinyl windows in a house once, and put foam between the bottom of the window and the sill beneath. I made one mistake. I didn’t lock the windows shut first. The foam forced the center of the frames upward enough that the windows would not close enough to lock. I had to dig out the foam with a sharp knife and start over.