I am fortunate enough to have a friend who is a carpenter by trade and happens to have a router which is perfect for the job of cutting a hole out of my bass for the pickup to fit into. If you are not competent with woodwork or don’t know someone who can do this for you I would suggest paying for someone to do this for you as you only get one chance.
We decided to drill the cavity as deep as the cavity used for the P bass pickups. If this is too deep it won’t matter as some of the space underneath the pickup needs to be filled with foam to keep the pickup in place and that can be increased or reduced depending on requirements.
Overall I am happy with the outcome. I have then varnished the new cavity to reduce the risk of moisture getting into the wood, Pictures below: