Rats, I measured that, and somehow forgot to add it!I think you'd better measure the diameter of the inner circle as well. If the reel doesn't fit the central tube, it won't be light tight.
It's an awkward contour, but I rolled up a piece of paper into a tube and pushed it through, and it looks like anything wider than 7mm diameter won't fit.
. Well, perhaps I should try the fridge trick mentioned earlier (and at some point get a new tank + steel reels).
.....just kidding. I use a spray nozle like the one on a kitchen sink.