This is a plug for many types of PTFE Fittings comonly used in dry boxes for 3d printing filament. It could also be used to fill other 4mm openings/holes in dryers, splitters and such. Don't worry if the post walls are a little bit rough, its the taper at the end that matters most to seal air.

It has a 20mm wide grip that easily prints without support. There is a 2.5mm chamfer to give the 4mm diameter post a little extra strength but to alo allow some room for you to get under grip to push down the lock on the fitting to remove it. This works wonderful and could be printed in any type of material. Total length is 35mm and the post with chamfer is 25mm. I left it that high to help with removal but you could always shorten it in a slicer.

The print profile is for Bambu PLA Basic, but any filament where you can get high quality walls on small diameter circles will work just fine. I use 3 walls so the post is solid. Infill is set for 15% Gyroid. No brim or support needed. There is a single plate to manipulate as you will or a preconfigured play for printing 5 at a time by object if you prefer instead.