Summary

Just another ballon knotting gadget to avoid hurting fingers... ...and fitting for my individual hand- and finger-dimensions ;-)
I got inspired by this https://www.thingiverse.com/thing:4919795 and this https://www.thingiverse.com/thing:4921077 (edit: and this one https://www.thingiverse.com/thing:3118389) plus seeing a kind of tool showing the idea tying a knot with a kind of tool at a friends house in action (I have no idea what all the gaps should do, for simple knots it seems it's not necessary...!?).
In case you want to change it, just use this link...
https://cad.onshape.com/documents/6b07c47e32a748dc79b9c187/w/75fb32fd176ac87f511accc0/e/93c965b199810df844cdb581
...to modify a copy to your needs. Tested with PLA and 100% infill (quite thin but stable enough, no PETG needed) and 0.1 mm layer height, just some minor support structures under the "finger-cavity" added.