I posted a thread recently looking for a source for a cap for my old aluminum rod tube, but nobody could find one, so was casting about for a solution when my eye fell on the small mayonnaise jar on the counter - lo and behold, the lid would screw onto the tube. So I just put a hold in the lid and ran some string thru it, attached it with Gorilla tape to the side of the tube, and screwed it on. Might work for you, if you have a tube with no lid.
As some one who has a rather large gap between the tailgate and bed of my truck :shock: I was very interested in any response to your original post
Maybe someone will chime in here, if not I’ll start thinking about this “outside the box (or tube)” myself
I lost the lid of my mayonaise jar and have not been able to find a source for one (except to buy a new jar). Went out to my garage and saw an extra rod tube and noticed the lid on it. Tried it on the mayo jar and lo-n-b-hold it screwed on. 
Congrats on your find.
Oh good one WC !
Cheers,
MontanaMoose
I hope you eye didn’t go down the drain.