This project builds a circuit to supply 5 volts to an MD portable. The power source you supply to it will be DC in the range of 7 to 12 volts, usually provided by a small "wall wart" DC supply.
OK, if you're going to go to THIS much trouble do it right and buy a 7805 regulator. Power to the left pin, 5V out on the right pin, and ground for both in the middle. Handles up to an ampere with a heatsink (not much without). Better run at least 7V in to be safe, it has a pretty high dropout (i.e., amount the input needs to exceed the output voltage) as regulators go, but it's cheap, reliable, and puts out an honest-to-goodness five volts. Available at Radio Shack, or ANY electronic parts supplier (this is a VERY COMMON part). Ideally you might throw a 100uF capacitor on the input and a few hundred on the output but they work fine without that.
You can buy a metal project box at RS too and use that as the heatsink, get a couple grommets...easy project if you have any soldering skills at all.
BTW, the input adaptor needs to be able to supply the full 800mA (or whatever the portable needs) current. Input voltage is not that important. Anything from 7V DC on up to, oh, I wouldn't go over 12 V in, would be fine. The reg can handle 30V in I think, but remember that all the overvoltage (times current) gets turned into HEAT. e.g. with 12V in, 5V out, at 1 ampere it'll dissipate 7 Watts. I think that's within spec for a good heatsink...with 7V in it would only dissipate 2 Watts.