Hi bonesbr549, There are a number of good options depending on your machine and the kind of work you do.
3/4 MDF using M8-16 button head fasteners in .5" deep pockets is probably one of the more common setups (can get extra M8-16 fasteners w/roll-in t-nuts from our store --> M8 Roll in T Nut | CNCRouterParts)
Here is a link to our reference spoilboard project --> Benchtop PRO CNC Spoilboard by CNCRouterParts - Thingiverse
This is presented here as a Benchtop PRO 2'x2' project, however the Fusion and Vectric project files are provided and can be adapted to the size of your machine (if you want to use them, take a look at the included how-to videos).
I'm hoping to see other replies, we always like hearing about what others are doing!
-Nathan