Not a sobering thought!

I guess the trailers you guys use in the US are made from aluminium, which is a shame otherwise you could use a sheet of that magnetic van vinyl and get a custom design made for it - stick it on 'magnetically' when you want to, peel it off when not and roll it up and store anywhere (I guess a sheet of steel glued to the inside of the trailer panel would also make it work)

If not how about a three aluminium chanels riveted to the side of the trailer (painted to match!) to make an open ended frame, that would allow you to slide a custom graphic into place when you wanted to.

Just some ideas.