I actually have been throwing together offset attenuators recently also. Just get a cheap 2 or 4 mhz oscillator module. Get a small box (plastic even, radio shack's smallest plastic one works well) and a few bnc bulkheads, and a bnc m-m coupler. Run the bnc straight through the box. Use a 50k variable resistor to control the amount of the 2/4mhz you are adding into the line as it passes through.
Then tune your receiver to +/- 2/4 mhz and follow that. 4mhz seems to work a little better, but some HTs have problems getting 4mhz away.
You can achieve at least 70db of attenuation with this setup. Unfortunately, those oscillators create a lot of power, (5v p-p typical) and it's not needed. It'll bleed around and through the pot and air-mix even when you have the mixing resistor turned all the way down. If you need more attenuation, you'll need to put the oscillator in a shielded area and have a carefully shielded way to switch in ~50db of attenuation on the 2/4mhz before it leaves the shielded area to go feed the pot.