Maybe get an EHX Screaming Bird Treble Booster and rehouse it in a crybaby enclosure?
I saw a lot of stuff online regarding modding wah pedals as treble boosters or tone boosters or eq pedals, but I mean more along the lines of exactly the same function the tone knob on my bass does, just as an outboard effect. So like I run the tone knob on the bass wide open, and then can roll it off with the motion of the wah. I'm thinking it should just be a pretty straight forward pretty much wiring the tone circuit for my bass into the pedal, and it would be passive, but I'm probably not thinking it through enough yet.
I think byoc and smallbear etc have wah enclosures and all the parts to do them up how you want.
That or find a cheap old Vox or crybaby. I got a model v847, very sturdy and easy to mod. Doesn't have a 9v out but you don't need one I guess.
Yeah, I have an old Bass Crybaby (not the 105Q) that I'll probably gut and rewire for the project.