I needed to put some grommets under the rear passenger seats to cater for the RG-400 Comms coax. It took quite a lot of figuring out on how to expand the holes without doing too much damage.
Eventually I worked out to use the countersink bit on a right angle drill, followed by a step bit. It needed to work that way as the step bit would not fit straight off.
Anyhow, now complete, ready for wiring. May need som adel clamps for managing the connection.