A construction epoxy Rezi-weld 1000
REZI-WELD 1000 Construction Epoxy , a product of
W. R. MEADOWS, INC. Construction Products will work without any problems. It costs about $20.00 for a large amount. I don't see a size on the box, but it consists of 2 parts A & B and both parts came in 2 quart sized paint cans. They may have been filled half way. It's an industrial epoxy so you may have trouble getting it. It's used for precisely that application. I had to use it to epoxy corner of cement that broke off. You will never get the plaque off later if you use it. It was available here:
Angerstein's Design Center and Building Materials � stores in Wilmington and Milton, Delaware � your source for residential lighting and building materials the last time I needed it.
It might be nice if you included the approximate size and weight of the plaque. Use a bathroom scale.
If you drill, and let's assume that the wall is a cinder block use a stud finder to determine where not to drill. You want to drill into the hollow portion. The only anchor I would use has a stud sticking out of it. The more you tighten the nut the more it clamps down.