You want to limit the bandwidth of your voice dude.
Cheap 16bit Analogue to Digital convertor then convert it back again with a Digital to Analogue converter. That should do the trick and make your voice sound awful.
Introduce a pink noise generator in there too and your laughing.
What you have to work out then is how to shut off the static when your voice isn't present. Otherwise you'll torture yourself. A comparator IC could be used there to toggle the pink noise on and off when your voice is beyond a certain threshold...?
I know you don't want to buy one but try this guy
John May, old CO of the UK Garrison. Hell of a nice chap and he's the chap who makes all the UKGs ROMFx boards. (which do what you want and beyond)
So basically, to get the more "movie authentic" sound from your voice amp, it's probably best to buy the pricier Hyperdyne rig than using a Radio Shack model, which you'll have to add several more components to work correctly? If so, I'm glad I haven't purchased a cheaper amp yet...
I have the ROM/FX Pro and it's excellent - I used it for my TK, TD and now my Fett and wouldn't want anything else - it's a great little board but I've upgraded with a Nokia car kit speaker as it's better than the one supplied and was only £3 on the bay of the E.
I also have the upload kit which allows me to change the onboard sound effects which also comes in handy!
If you want a CEAP one, go to Wal-Mart in the novelty toys area. They have one for $6 that does a monster effect, alien effect, and a plain voice amp, to do the "static burst" just do a ssst sound and it's works great. Your voice can be clearely understood, and it's CHEAP, who needs them $100+ voice amps, go walmart yay!