Those larger lights incorporating an indicator are still used by trucks, and for some vehicles in the Kit Car world.
There are two sizes, both readily available, I believe the manufacturer is Hella.
I have heard of bustle racks being fitted with cycle type LED's as markers.
Personally I will go with the trailer board idea, without trailer-triangles, as it is easily removed and highly visible,
and the GPW is fitted with a hidden 7pin socket, with indicators,
For my MBT I have a pair of those small Hella combination ones,
You can't be too careful today, as modern traffic moves so fast, and they easily underestimate your speed (or lack of it!)
Exeter Devon UK
FORD GPW 202055 DoD 5 - 22 - 44 Serial# 20534360
MBT # 25553 DoD 4 - 43
MATCHLESS 1944 G3L C5806205 (Ex 33 ZC 80)
Piper Cub 1944 L4H 480015 G-AKIB
1942 CHEVROLET G506 CARGO. #2NM 213219