Here's another resource - actually this is the one I was looking for originally, but was at work so didn't have much time to look further:
https://groupdiy.com/index.php?board=29.0 To keep costs down, you could look at using your big diaphragm capsule with different PCB circuits. The PCB circuits should be pretty cheap to make, assuming there's no transformers involved. ProtoType board or stripboard should suffice until you get a circuit you like.
But don't write off your existing smaller diaphragm capsule. Using better grade capacitors, or even trying out a different circuit altogether could radically improve/change your sound.
[Edit] - meant to add... if there are transformers involved, make sure you stick to tried & tested circuits / component values as there can be some crazy voltages involved. The last thing you want to do is fry your audio interface.