Done it, works as well if not better than some commercial diffusers. I personally believe the only reason someone should spend ludicrous sums on cheap plastic is if they believe their clients will truly judge them on what they look like rather than the results they produce.
The other good one to try is using a sheet of injet paper cut as a trapazoid and rubber banded to your flash as a forward reflector. That's my favourite type home made flash modification.
Garbz - that's true for the most part, but sometimes I like to pay for the thought that went into it; someone took the time to design it and now they're providing it with good quality material (sometimes not always just cheap plastic )... if it's not outrageous, I'll buy it.
just google it, theres plenty of ways to do it and very easy to find if you just do a quick search. theres a couple of different ones, and some other pretty neat DIY stuff (mostly lighting related) at DIYPhotography.net