Did you add a block of epoxi or there's metal inside?
It's grey epoxy putty...I think Oatey is the brand. JB Weld makes there on version too. It's a plumbers material too. Two part solid putty mix that u knead and form it into the shape u want. Just smash it down first on a roughed up spot and then bury a threaded Tnut insert in the middle. After it hardens u can shape the mound into a block. I've done it myself or just buy some Plexus and do the same thing. Both work well. Fast set epoxy with chop strands will work too.