I used a rubber sheet cut to fit behind the flange with the hole for the pipe barely large enough to fit snugly around the pipe. Most people cut an X smaller than the pipe diameter since the rubber will stretch to fit.

I used a silicon type adhesive/filler to attach this gasket to the filter body although there are a number of products that would work as well. It doesn't have to dry hard. In fact it is better if it remains flexible. It has been in place over 10 years now. The flange can be tightened down to compress the rubber to prevent leaks.

Check the plumbing section for DIY rubber sheets. They are usually about 4x4 inches.

Sleepless, couldnt you use a scrap piece of EPDM, or is the rubber you are referring to stretchier than that? Thnx -I am about to start my own plumbing project for my filtration and I have a lot of leftover pieces.

The rubber is much thicker than the EPDM. It also is textured on one side to hold the filler in place. It's cheap enough that I didn't try the EPDM.