David, you have done a really good work.
Its funny to see the way a electronic guy is solving problems aut of the view of an mechanic guy.
(I'm an machine fitter / draftsman / CNC-machine programmer)
For example:
You said that aluminium parts are cheaper then steel parts.
Thats only the case if you mill the steel parts.
If the parts are laser cut, steel parts are much cheaper.
Because steel sheets are less expensive than aluminium sheets.
I know here a company, which lasercut the parts and make all the threads on the same machine.
If you have problems with harmonics, bend the part on the sides or screw ribs on it to lift the resonances over the hearable area.
About the zinc plated steel plates:
Your's are "blue" galvanic zinc plated steel plates.
There are yellow and black versions of galvanic zinc plated steel.
The funny pattered coating mentioned in the video is a hot dip zinc coat.
Its much thicker than galvanic coat.
Its used at low precision parts like fences, suport beams,...
I you buy zinc coated sheet metal, its hot dip zinc coated too.
Its funny to see that the electronic guy moan about countersunk screws. (I'm moaning about getting matching resistors, capacitors,...
)
Nearby my home, there are many shops for industrie suply which sells all sorts screws.
And that to a better price and quality than at ebay.
Dont go to a DIY superstore. They sell the screws with bad quality at ridiculous high prices.
By the way:
I dont use the countersunk screws with philips head.
They are a pain in the ass if you have to remove them after a couple of years.
I always use screws with hex-head or torx-head.