Yeah...I built one out of a cheap Radio Shack sub.
Not hard at all. The hard part is mounting it. I happened to have one of those adjustable armed snare stands that I collapsed to where the three arms cradled the speakers magnet, then zip tied it in place.
I took one of my XLR cables, chopped an end (make sure you pick the right end

) and wired the + of the cable to the - on the speaker. Vice versa for the other and then took the common ground (probably just copper wire) and wired it straight to the speaker chassis. Take a file to the paint on the speaker if you need to...just to make sure you get a good connection.
One other thing that's a "must" is an inline pad. Maybe called an attenuator. I found one from Shure that's adjustable from -15, -20 and -25. The signal coming out of the subkick is HOT. You'll want to damp it down before it gets to the preamp.
I think I might still have some pics around if you're interested.
Peace.........Kel