I was too lazy to implement
Kom Alakdan's HLK Drum Mag Tweak so my HLK became a casualty of the Yarik Kalsador's ROF tests.
Luckily, I found this piece of aluminum in my junk box.
We will cut that piece along the red lines in the pic above so that it can sit on top of the drum mag to provide a replacement lip.
First, we smoothen the aluminum piece with good old epoxy (left); then we cut off an appropriately measured part of the aluminum bit and carve out the green and mustard bits marked in the pic below (right) so that it will match the internal contour of the forward part of the mag.

It fits!
After gluing, filing, and sanding to make it fit the Kalsador magwell, it will be ready for a motor and, probably, installation of a switch, some contacts for trigger activation, and, finally, painting.