The first concept was to 3d print a threaded sleeve to glue to the outside of the top of the barrel, with a matching cap equipped with a tank valve (design pictured).  This was rejected for the following reasons:

1. The temperature resistance of the plastic and/or glue might not be sufficient

2. Modification of the aeropress would be permanent.

3 - Online 3d printing services are surprisingly expensive.

A simpler alternative using readily available materials and minimal construction effort was preferred.

A cap is made of a piece of plywood with a circular groove routed in it with a desktop cnc router or just a router table.

The groove is sized so that the top of the aeropress barrel fits into it and seals against an o-ring.

The o-ring I used is as follows: 228 Buna-N O-Ring, 50A Durometer, Round, Black, 2-1/4" ID, 2-1/2" OD, 1/8" Width (Pack of 10), <$20.  These have actual width of 0.139".

The groove is 3/16 deep and somewhat smaller than the o-ring width so it doesn't fall out, say around 0.133".

A 1/8" NPT tank valve is threaded into a hole from the other side of the plywood.

Threaded rods attached to a base are used to clamp the cap on top of the aeropress barrel.

A cheap bicycle pump is used to pressurize the air space.

The coffee is expressed in less than 10 seconds with minimal effort.

I use a very fine grind (~5 clicks on a timemore chestnut c2).