reflector tape

30 years ago had a Komfort TT that had a whiter elective tape running the length on the sides just about headlight height. It was about a 1/4" high, blended in during the day and did a good job when headlights caught it at night.

I also marked the corners of my slides with tape when they are out. Makes me feel better if they are out at night, say at a Wally World.
Catfish

TSI carries reflector tape as well. I used it on the back bumper of the TT.

Harbor Freight reflective tape.

We got ours at WalMart.

Can get at NAPA or truck/trailer repair business. If unsure, call local Ryder or Penske rental shops for closest place to buy. I bought a 100' roll and have nearly the whole thing left still after doing my snowmobile trailer all the around.

I got a package at HFT, put full width on rear bumper.

Check out industrial safety equipment. I have part of a roll designed for commercial warning signs. VERY reflective, super adhesive, weatherproof. Beware though, it is almost impossable to remove!

You can buy like DutchmenSport at Wal-Mart

Keep it simple, keep it clean, keep it recognizable for the drivers behind you. Something easily recognizable, like this.