So I ran off and bought a pressure gauge, elbow and adapters to piece together a portafilter pressure gauge for myself.
When I got home and put it all together, I realized (eventually) that my current configuration of: basket -> portafilter -> 3/8"x3/8" elbow -> 3/8"x1/4" adapter ->1/4"x1/8" adapter -> 300 PSI gauge just wasn't going to work.
I had completely forgotten that I do not have a seal between the basket and the portafilter itself!
Lets not worry (for now) about the fact that I have forced together the 3/8" parallel threads of the portafilter and the 3/8" NPT on the elbow.
Does anyone have any advice to make a seal between the basket and portafilter?