Matte boxes mount onto iris rods and the front element of the lens slips into them through the donut. The donuts are in varying sizes so you can use whatever lens you want with the matte box. To account for different lengths (physical, not focal) of lens, you just adjust where on the iris rods the matte box is sitting.
They also range greatly in price, so that will likely be your deciding factor. The "cheapest" matte box that is still worth using probably comes from RedRock. Anything cheaper is likely to be crap. There's a chance you could find a better one used at a similar price to a new RedRock matte box. I am also often proven wrong on this sort of thing, so maybe someone else knows of a budget matte box setup that is worth using.