Tom order a square foot of a 1/4 inch aluminum plate on eBay, I'll send you the template and if you've got the time get a fine edge jigsaw blade and go for it that's what mine are from.
Guys,
SendcutSend dot com is a good outfit. You can upload the file and they will tell you what it will cost. They supply material and send you finished parts.
Tom 6061 is a common billet compound and I have used it a thousand times for projects in the past, I still have a 4x4 foot sheet of 1" here at my house even though I do not have a waterjet anymore. Yes I would use 1/4" material, its a standard thickness and will do the job easy no need to go thicker.
What type of Hardware are you guys using to install these corner brackets. I'm unsure what structural material is behind the skin at the corner of the HWH slide outs.
I was thinking #10 or #12 X 1" self taping screws
Great idea!!! I need these! I wonder if I could 3d print in a material that would be strong enough - maybe carbon fiber? I could cut them in my CNC also…