You can get one off a P68 magnum and cut it down, cut just behind the feed port. This will give you the dove tail to mount the red dot.
There are plenty of them out there with a broken feed neck that can be had for around $10. They are aluminum and can be cut with a cutoff wheel or a hack saw.
look here:PMI magnum feed block, used shape with copper feed - Sheridan feed necks (solder+pvc) - Sheridan - Pump Parts - Parts