#1 Drill the Plate and screw from under side.

#2 Drill thru the upper extrusion and bolt into lower T-slots. Can add some Blocks into the slots to stop them sliding.

#3 If just bracing then 5mm steel will be just about ok, But personally I'd use 9.5(3/8")mm Aluminium so don't have hassle of painting etc plus steel on ali is not good mix.

#4 Can do it but it's difficult and not very effective unless high vacuum. Much better with Blown air and minimal Mist to clear/Lube chips.

#5 Simple holes at one end and angle the machine on bench using feet.