I was trying to find 300 x 100 x 3.2mm ali box section but no one does it that size in ali only in steel.

machining it out of 1.75" ally plate is a possible, but its £25 worth of ali. However if you plunge cut it all the way through with a 10mm cutter at least you'd end up with an approx 280 x 80 remnant which is useful; then make top n bottom out of 4mm plate.

Another option is to make it up from 4mm plate and weld up the corners... then machine clean