I have a switch box of sorts. I made it as an input selector, for when I had a speaker amp with only one input.
The box, and bits, cost me less than $40, and it wouldn't be all that much more money to make a slightly different design (of slightly higher quality components) which would be designed to act as an ABX test box.
You'd probably want the box to sit connected between the source and amplifier, with both sets of cables connected to it at both ends, and a toggle switch, so the signal was only going through one pair at a time. For this, all you'd need was an enclosure, a suitable switch, 4 pairs of RCA sockets, and some hook-up wire (suitably high quality of course, so that it's integrity was not called into too much question

).
You could also do it cannibalising an RCA lead, and having two short "pigtail" sections, that went towards amp and source, so that the distance of "non-test" cable was kept as short as possible...