I finally got around to making one a few weeks ago. It uses non-inductive high power resistors. There are actually two separate attenuators in the housing - one for 8 Ohms, and one for 16 Ohms. They can be cascaded to give four levels of attenuation.
Initial tests are good. No discernable loss of tone.
On the "HIGH" setting it drops a 60W amp to 10W.
On the "LOW" setting it drops a 60W amp to 2W.
Can be used with amps up to 100W.
Total parts cost about USD$150. Time to make ... about 8 hours

. .