I wouldn't try to make t-slots in the wood. they won't hold well.
Either bolt down a sheet of OSB or flat plywood and sink the hardware (I like M6 flat head screws for this) deep into the wood so you can resurface
In this scenario, you won't have t-slots to work with, you'll have to screw material down to the surface board
Or rip strips of wood that are wide enough to get 2 sets of screws into them and they expose t-slots on either side of them - basically 3 strips wide minus t-slot widths. bolt them in and then you'll still have more than half the t-slots accessible.