Don't bother.
A) A fair number of Pis are run from an USB port (yada yada, don't do it, whatever). They're thus directly connected to the PC's +5V rail thru a simple polyfuse.
B) Remember that fuses are not instant, magical devices. If you're trying to protect the Pi, a fuse alone will do squat, as it's too slow to respond. You will need a voltage clamp after the fuse.
C) The Pi is, as you said, $35.
Lastly, consider the fact that if you have no other loads for the PSU, it will likely - if it's as crusty and old as I imagine it is - eat 10-20W by itself; the quiescent power loss is so high you could run several Pis on it. Just get a cellphone charger rated for a couple amps (... but not a shitty deathtrap from china).