100 mm aluminium what you want?
A monster in your house , i ts not gonna be cheap too.
Mine are 40 mm which is more then enough i think i could go to 30 mm also without problem .
Remember magicos extruded S 5 aluminium is only 14 mm iirc.
Cast not milled. You can get a custom-cast piece in Germany for not that much money. But that in 5 years time.
Right now I'm optimizing driver-radiation patterns.