Unless you've got another idea it's gonna have to be Plate, 6mm will do & last if it's done right
I take it you don't have access to a CNC or a Waterjet so your looking at cutting it out of Plate with a Plasma, either or, it's still gonna have to be Plate.
I could be wrong, but It's not going to be easy & after all what else are you going to Weld Steel pipe to.
Been following this Fred right from the start (along with Stu's), very interesting & all the best to both of you, ti's an inspiration, A tell thee.