Does anyone know how to build a small mid boost circuit that you can fit into a guitar's control cavity? And if you know how to build that. Could you also tell me how to build a similar volume/ gain boost circuit for the same purpose?
For the mid boost, i would use a simple op-amp(like a TL071) with a bandpass filter or alternatively a low pass filter fed into a highpass. I do not have a schematic but it should be simple enough for anyone with electronics knolage to draw out.

For a volume or gain boost, do you simple want increced output, or a small distortion circuit. A volume boost could simply be another TL071 op-amp fed into the volume pot.

Consult your nearest analogue electronics textbook containing information on OP-AMPs
Gear List:
Ibanez S320
Lavey LV200
Squier Stratocaster Affinity HSS
No-Name Wesley Fat Strat copy
Peavey Rage 158
i just found this site, which is absolutly fantastic for designing a midboost circuit. basicly it lets you design an active bandpass filter.

as for a simple amplifier, thats just going to require an op amp and two resistors. dont feel like doing a great drawing but heres the idea:

    | \
in--|+ \______out
  __|- / |
 |  |_/  Rf
 |       |

where the output is out=(1+Rf/R1)in
so you make Rf a pot, and have R1 fixed at a number depending on how much gain you want. experimentation is good here. probably not the best boost circuit, but it should work.