TerribleTortoise

This needs a “reticulating splines” step.

I like to recommend the episode(s) on John McAfee when people first start Bastards. They’re wild and not nearly as dark/heavy as some episodes can be.

If you want to be able to trim off the ends, I suggest a double fisherman’s instead (aka the double overhand stopper) with both strands (hold them together while you tie and treat them as a single strand).

https://www.animatedknots.com/double-overhand-stopper-knot

With either knot, after you trim, you can melt the ends of the elastic cord. It will “mushroom” a bit, which will also prevent the tips from creeping into the knot and coming loose.

A flat overhand bend in the elastic cord is what I recommend: https://www.animatedknots.com/flat-overhand-bend-knot

Justification:
Simple, not bulky, reasonably secure against shaking loose, and relatively easy to untie.

As suggested in another comment, a double figure eight could be used as well. It will be more bulky and a bit less likely to come loose.

That’s a Highland. A Scottish breed of cattle.

Affectionately known as a “Heilan Coo”

For science: https://www.sciencedirect.com/science/article/abs/pii/S1752928X17301038?via%3Dihub

Edit:
You can’t just post something like that and not a source! Citation required!