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.