The fact that the Boss controller for the katana head (ga-fc) costs around £80 was enough incentive to build my own controller. Casings are hard to find, so I bought a digitch control 7 on ebay for about £13 and wired an arduino nano into it. It already had a midi socket, but the display used 4 individual 7 segment displays, so I swapped these for a single 4 digit display (TM1637) which only needs 4 connections and costs a couple of quid. The coding took a little longer, since I wanted the display to reflect the FX1 and FX2 setings, but I got there in the end!