has anyone built a browns gas generator?

The mathematics and science says it doesn't work, don't wate your time. It's a very persistent con. I've NEVER seen proper peer rieviewed & repeatable testing to prove they work-because they don't/can't help you. you'd be far better off using a Gasifier as in WW2 cars in UK, Europe and Australia. They convert carbonaceous materials into various combustibles by partially burning them at hi temps with low oxygen. Modern variants.are still used in some areas, not perfect solution, but prob better. LPG conversion is better still, tho' more dangerous than petrol. Hydrogen fuels are suicide-it's explosive at any concentration with O2. Thus motor destroying too.

Hydrogen is not much more or less dangerous than any other flammable gas - used appropriately. A unit such as you are looking at will make zero difference - just a wind up and a scam. You cannot make meaningful amounts of the gas with out using several thousand watts of power - remember that a petrol 2 litre engine making a hundred thousand watts of power is not even going to notice if you add few cc's of gas to the mix.