Following the idea of Headphone Dummy Load by Warren Young, I've made my take on implementing it. I noticed that Sparkfun's Mini Solderable Board fits neatly into Hammond box 1590A recommended by Warren, and decided to use it as a base for the assembly. Here is how the soldered board looks like:
The wires of the load impedance switch are soldered to the board, while the input cable will be connected via the blue terminal. I think there is an advantage in using the terminal: first, one can change the cable without desoldering; and second, it's possible to attach probes of a multimeter to the screws of the terminal, which can be useful if the amplifier under test is not disassembled.
This is how I prepared the enclosure box for fitting the board assembly inside:
After mounting the board inside and screwing the resistors to the enclosure I ensured that the board sits very firmly. There wasn't even a need to put a second pair of nylon nuts on top of the board--the latter is held in place by the resistors:
Overall, this was an interesting and useful project, although not very cheap. But the box is definitely more compact that 2 pairs of headphones, and also the device should be more reliable in withstanding accidental high voltages or currents from amplifiers under test.
Below is almost complete list of Mouser parts I used:
Enclosure: Hammond 1590AGY (546-1590A-GY);
Rubber grommet: Heyco G1019 (836-G1019);
Breadboard: Sparkfun Mini PRT-12702 (474-PRT-12702);
33 Ohm resistors (pair): Ohmite TCH35P33R0JE (588-TCH35P-33E);
330 Ohm resistors (pair): Ohmite TCH35P330RJE (588-TCH35P-330E);
Switch: E-Switch 100DP3T1B1M1QEH (612-100-H1111);
3-pin terminal: Sparkfun PRT-08433 (474-PRT-08433);
4-40 screws (0.25" for resistors, 0.75" nylon for the board) and nuts.