David,

I dont know that the behringer looks like inside, but, what about using its case and replaging the jacks with 3.5mm ones

Depending on how large the holes are cut, the cliff sockets may fit nicely with just a tiny bit of tape packing (have swapped 6mm
for 3.5mm on oakley pannels like this), but i suspect the inside is basically a pcb... Possibly stripboard would work out for you
inside Or even, and its getting scary now, wire

I know itlld take a bit of time, but ive seen those bx2000 (or whatever theyre called) selling second hand for a fiver sometimes..

Julian