Haven't seen this anywhere so thought I'd post it.
Needed to use some rust removal liquid on the fender stays and was trying to find some sort of long thin container. Was looking for some hose I could cut up and noticed a bike tube.
Cut the tube, rinsed the talc out, folded the end over several times and clamped with a spring clamp. Clamped the fender in the bike stand so the stays were vertical, filled the tube with rust remover and slid over the stay. Clamped the tube at the top of the stay with another spring clamp.
Used a drip pan since the tube overflows when you put it over the stay.

Think I'll try the same thing with a mountain bike tube and a handlebar I need to do. That should be easier since I can add the rust remover to the tube with the handle bar already in it.