Building your own Boss FS-5U footswitch Edit
Several Boss delay pedals (DD-7, DD-5, DD-3 e.t.c) have a jack that can be used to connect an external footswitch to manually tap in the delay frequency, instead of having to lean down and twiddle the knob. The recommended footswitch is the FS-5U which retails in Australia for $59. Fuck that, it's a glorified switch, I'm building my own.
It turns out all you need is the right switch: a single-pole, single throw momentary action. From Jaycar try the SP0702, although the construction is a bit wimpy. You'll also need a standard 6.5mm mono audio jack.
Total cost: $5.90
For the case, I used an empty tuna can, although you can use anything you want.
Make holes in your casing for the jack and pushbutton. Drill a guide hole and then widen it with a big screwdriver and a hammer. Or just go straight for a pointy thing and a hammer. In a well ventilated, spraypaint the casing to a colour of your choice.
Next is the world's simplest wiring: connect each pin on the switch to a pin on the jack. That's only two wires and the order doesn't matter, so you can't screw it up.
Extra-credit: Adding a de-bouncer Edit
I started designing a de-bouncer for the switch, consisting of a 0.68uF cap in parallel and a 18K resistor in series on the switch side of the capacitor. I actually determined the Thevenin equivalent for the DD-7 footswitch jack for future reference: 2.11V (tip positive) and ~300K. The debounce filter would then have an 'on' time constant of 12ms and an 'off' time constant of 20ms.
However I didn't actually have anything approaching that value capacitor, and it seems to work fine without it (I figure the engineers at Boss put debouncing in the pedal). So this left as an exercise for the interested reader.
Finished Result Edit
Works well enough for tap-tempo on my DD-7. Pics: