Creating/making a naked portafilter is very simple using a hole saw. This how I made mine.
-Dremel and grinding stone or sand paper
Step 1: Step 1
Use hole saw to drill out bottom.
Step 2: Warning
Watch out for the small shaving, cover your eyes, hands and don't breath it in.
Step 3: Step 2
Smooth and clean up using Dremel and grinding stone
Step 4: Step 3
Clean well before using