But then again, I make lots of 40 point landings and get 28-32 points for them.
I have that problem, too. Particularly when you're judging.
When you're experimenting, try using a bit of mild steel, like from a tin can (a pineapple can would do). Then locate a 1/4" or 3/8" diameter neodymium magnet close to it. If it works you'll have a hysteresis brake. Assuming that you can make the disk run true, you should have a pretty consistent braking, but I'm not sure if it'll be enough. You can then adjust the amount of braking by the spacing to the magnet, and possibly the number of magnets (alternate them N-S-N-S).
I'm not sure if the hysteresis brake will work -- you need steel that's magnetically soft enough to get magnetized, but magnetically hard enough to hold onto the magnetization. If it's too hard the magnet won't affect it; if it's too soft it'll let go too easily. Some experimentation may be in order -- fortunately, for plain old carbon steel the magnetic hardness roughly tracks the physical hardness.
If you use aluminum or brass instead of steel you'll have an eddy current brake. It'll slow you down fast at high speeds, but not give you any braking at all at low speeds. A hysteresis brake should work all the way down to a stop, although at high speeds it'll probably have an eddy current component as well.
Edit: You probably need to do some lathe work to get the disk running true enough. I don't know how true it needs to be -- just give it to Haverly and tell him that some engineer said it needs to be "close".
More edit: And don't blame me for the wad of rusty washers, cotter pins, and iron-bearing sand gumming up your wheels.