Many people who own the Aeropress asked for a specific item: Tube to hold ground coffee. This fits fairly snugly.
You should probably use food-safe materials. The tube prints well with a .4mm nozzle and well-adjusted flow. PETG with no cooling works well for the tube, and may work for the cap too. I printed the cap out of soft TPU. The cap has to flex a little bit, so don’t use too many walls for it.
Scale down to 96 to 98% if you’d like a slightly looser fit.