It will maybe an old technique but I was designing such filters using with EM coupled high Q values ( silver plated ) mechanically tunable inductors or fixed inductor and a piston trimmer variable capacitors
Because here the coupling is very important.It won't be neither high nor low.I obtained few MHz Bandwidth but few kHz is roughly possible..
In fact, 40MHz is a similar to Amateur Radio Communication Band so if you search Amateur Radio Enthusiasts' websites, you will find lot of docs to be inspired.