I don't know who needs to hear this (the answer is probably no one Cameron) but the easiest way to close the end of IC tubes like this is to just take an xacto knife and make a tiny cut like this. Then you can press in or out the corner to make a toggleable door, no keeping track of little plastic pegs or tape required
@old_angry_queer Thank you for this. Just a touch easier than designing and 3d-printing replacement pegs, which is what I would have done.

I would have tried to squeeze a small amount of toiled paper or cotton into a ball that seals the tube.

But @old_angry_queer's suggestion is: Neat!

@janschiefer