When googling about this subject I came across alot of neat stuff.
I saw a Jag shaped 6-string bass from the sixties with hidden pickups.
Some succesfull attempts include the John English one with the (custom) Texas Specials, an old DiMarzio PAF, a 9.5K ceramic P90 and a builder of relic Fender copies. Unfortunately that builder doesn't reveil any secrets.
I used to have quite a collection of stock pickups. Some were butchered for parts and some I threw away. I shouldn't have done that. I could have tested them all to see how strong the pickup should be.
With Fender single coils what would help more? Tall magnets or wide magnets?