Hmm, just found my length of 8mm beeline and it is more like 12 to 13 feet in, but not every time.
It seems the problem is it goes in randomly, crossing itself and building up height of the reeled line quickly till it hits the outside of the shell.
Maybe we need a friendly designer at a plastics plant to make a custom-sized reel.
I may trace out the profile and make an extender to go between the halves, extend the central post, and see how that works.
Maybe another 3/4" of thickness. Originally designed for 20' of cord, so the spring take-up should be there for more length.
It may end up being too wide and hang up on branches and limit movement.