Boundary Condidions (BC) for Shake Table

Hello. I am designing a electronics chassis that is going to be tested on a shake table. My issue is what are the proper boundary conditions?  I have been doing this for a long time and before I just constrained the bottom surface of the chassis. I have gone through several different variations of BC's  Now, I just constrain the bolt holes that are around the edge of the chassis. I am still not happy/comfortable with them. See attached picture of model.

Anyone else have any experience or thoughts to the proper BC's.?

Thanks in advance