This is an inspection port I made for my Avid Flyer. Inside diameter is 3.54in which is JUUST big enough to fit your hand into.
The file consists of three parts - an inner flange, an outer cover, and a cutting template.
I printed both the inner and outer parts using 100% fill with Bambu's PAHT CF-filled nylon. The inner flange is meant to accept 6-32 inserts of the Amazon generic variety.
I printed the cut template out of PLA for a quick print. Go ahead and tape it in place and use a VERY sharp knife to cut the circle out. It should be 1mm diameter larger than the inner flange's rib which will protrude through the hole. The inner flange can now be inserted through the hole by bending it - the split on the inner flang should make this easy. Don't forget to pre-apply glue on the inside of the fabric and the flange.
Now glue the flange in place, cut the holes out (bonus points if you use a sharp hole punch) and you should be able to install the cover using 6-32 screws.