If the iris is pressed inside the 1.25" barrel until it is touching the retaining ring with the bright interior, it will probably reduce the brightness in that ring still further since the light would have to be even farther off axis to hit the retaining ring at all.
That's what TeleVue did.
I bet you could find a suitable rubber or plastic washer at a hardware store to do just that.
Or just cut a disk of black foam core - painted flat black or covered in ProtoStar - to fit snuggly inside the 1.25" barrel. A disk cut from a plastic milk bottle would also work.