The original used a dual concentric pot for the Boost/Cut and Frequency controls, but I think they are almost certainly going to be special order as one side is 50K reverse log and the other is 100K dual linear. So maybe just using two pots would make more sense for the average DIYer.
The trimmer is for volume and is designed to be pretty much a set and forget. Set it for unity, or a slight boost or whatever you favour and then close up the box. Again though some people may prefer to alter it slightly, do away with the trimmer and have an external volume pot.
Make sure you use 25V rated caps if you want to power it with 18V.
Info about the original:
The new Bass Mid Control is the active mid-controller for bass players. A small but powerful pedal that provides frequency control adjustments between 120Hz ~ 3kHz. When using 2-band EQ or passive bass, the Frequency control provides excellent manipulation of the mid frequencies, while the Boost/Cut gives you boosting or cutting capabilities of up to + / - 12dB. This compact pedal can also be powered by a 9V battery or for additional headroom use an 18V power adapter. This pedal will provide you the flexibility to play any musical style.