I'm going to agree w/ Crater here, and add that boost makes things more complicated because it almost requires active circuitry.
Also, a caveat: 6 bands is alot of knobs to fit on one box, which is why EQ's usually use sliders, but those are usually harder to find and more expensive.
But all is not lost if you really want to do this:
The simplest "EQ" you could build would be a low pass filter(essentially the tone knob on any passive bass). You'd need:
A footswitch(if you want to turn it on and off...)
2 1/4" jacks.
It's easy to figure out the circuitry(look at a P-bass schematic and learn about basic passive filters). I'd recommend you build it first(remembering that you want to expand it, so don't drill any holes you don't want later), just to get some soldering, drilling and basic circuitry under your belt. However, it's not very interesting, nor is it really useful.
So, you could do an adjustable band pass which might qualify as a 2-band. The above would just be an RC low-pass circuit, but you could add in an RC high-pass filter. So 2 bands, 2 knobs, and an extra capacitor. More useful, but not amazing.
It gets interesting when you want to learn about boosting. You could stick w/ the two band boost and make it active, but to be really fun would be a 3 band. You'll need another knob, several op amps(or a dual or quad chips would help here), but most importantly, you'll have to learn about op-amps and feedback networks. There are volumes written on this, so start reading around.
The first two circuits could get you started quickly and you can keep adding to the original so you don't have to keep buying boxes and knobs.
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