100ml Measuring Tube Bung
A simple bung designed to work with this 100ml measuring tube:
https://www.amazon.co.uk/gp/product/B094D292PT/
I'd recommend printing this out of TPU 95A with around 25% infill. It's helpful to not have 100% infill as it makes it a little softer and more flexible which is what you want from a bung like this.