To mount the capacator plexi assembly, a sheet of plexi the size of the base will be screwed down onto the truck's floor. The cap assembly will then be glued to this sheet of plexi.
The smaller countersunk holes are for the mounting screws. They have to be countersunk so that the screwheads do not prevent the cap assembly from sitting flat on this sheet.
The larger holes are to allow the nuts from mouting the cap and distribution block to go through the mounting sheet. Again, the cap assembly's bottom sheet has to be flat against this one for this mount to work.