I've been using the great selector mod that vintagepc created for a while now. It's been working out very well for me, however occasionally the filament would shoot out the opened area just past the metal ball (pro tip - the metal ball that comes with the MMU2s isn't magnetic - I recommend replacing it with something like these https://www.amazon.com/gp/product/B01M67WRVT). This is a common problem I've noticed on all of the modified selectors that have the canal opened where it's too easy for filament to exit the selector inadvertently.
I simply imported vintagepc's selector in Tinkercad and added a curved wall right where the filament would shoot out on my printer. It's still opened enough to easily see what's going on in there, but closed up enough to (hopefully) prevent filament from shooting out the side of the selector.
I hope this helps out someone else who is having a similar issue!
The author remixed this model.