You don't need to go fancy with it IMO. One way, would be to grind it out from the backside, use a die grinder with a radius stone in the nook and crannies. Find a flat sturdy work table, lay up a couple layers of 1.5 oz. matt and lay the pre fit core into that. Put weights on top of the core so it squishes the resin and air bubbles out from under. Let it kick and then touch up grind the edges and glass the back side with a couple layers of matt or one layer of biax with the matt stitched on. I like to rough up the plywood core to give the glass something to bite to and put a smooth radius on the corners to help the glass lay better around all the 90 degree transitions. I didn't follow the Wellcraft technique of using small squares of plywood. I left the core in large pieces, drilled 3/8" holes in the core about every 4" to help the air to escape from the bottom side. Most of the holes had resin coming out after the weight was applied. I used composite so I was not concerned with any voids, if you drill the holes and use plywood I would make sure they are filled before you lay the glass on the backside, the less voids the better. It is a resin rich technique, but tough as nails and it works.
