You already said you used a mill, but ok.
If you know how to make DXFs, you can draw the bolt pattern and use a service like send cut send to have a part made for you.
I'm saying this because I'm not sure I'd trust epoxy or anything else to seal that fluid inside, unless you can find a single...
I feel like your adapter design leaves something to be desired. You've effectively eliminated two mounting bolts right?
Why not CNC plasma cut or laser out a 30mm thick plate that's got your bolt pattern and cavities for the fluid?