I've spent money upgrading to clamping feednecks. Honestly, I was happier with the results of sanding down the hopper neck to fit very snugly in the non-clamping feedneck. Worked in the early 2000's; still works today. Use 60 grit to get close. Then switch to 120 grit or finer. Just wear leather gloves, or you'll get blisters.
Expect sore forearms the day after sanding.
Support the hopper rigidly while working it into the feedneck, so you don't crack plastic. Spin it clockwise (righty-tighty) when installing, so you don't unthread the non-clamping feedneck.
Don't lose hope if you sand too far. A piece of electric tape looped over the top of the feedneck does wonders. I still do this with a non-clamping feednecked Autococker. My opinion is most people do it wrong by putting the tape on the neck of the hopper.
Last of the Salzburg Clan