What does the stainless steel plug do in the E3D Kraken cooler block?
The connecting channel between the two brass barbed hose fittings needs to be drilled out.  To open that channel, a hole is drilled into the side to connect the two top holes (which have the brass fittings screwed into them).  The hole doesn't need to penetrate both sides, but it must get through one side.

The grub screw seals the hole.

Instead of using epoxy to seal in the screw, you could use the PTFE (Teflon) white plumbing tape.  It is specifically designed to seal threads.

If I remember, you can thread it in as tightly as you like.  It won't interfere with the water channel.

I have a Kraken, and I looked at how to make another one.