I have a Miller Compass 12 fluid head and want an inexpensive sliding base plate that fits the Miller groove that I can permanently epoxy to a Manfrotto 394 quick release. That single 3/8 screw attachment just isn't stable enough when I mount my slider to the tripod via the quick release and I don't want to sacrifice the crazy expensive Miller plate.

I guess not then?

Just a thought, since I don't have that set up - something I've occasionally done is to drill and tap an additional hole to fasten the plate and QR clamp together. If done carefully it doesn't "mess anything up". But I'd have to see the actual parts to know if it would work in your case. Not sure I'd trust epoxy without some mechanical reinforcement. I've had epoxied joints fail in other applications. My clients used to use epoxy in some extremely high precision assemblies and they categorized the epoxy mixing and application as a "highest skill level required" operation. Improper mixing, cleaning, and application were real problems until they developed a program to train and certify the workers who did it. It isn't magic goop that can just sort of be aimed in the general direction of the problem.