You can use either aluminum or plexiglass. I used .072 aluminum for the template, which was originally going to be the panel. Aircraft grade aluminum is hard to come by in small quantities. Every metal supplier I went to would sell me no less than a full 4 x 8 sheet. I finally found a supplier who sold in small quantities, but he's since gone out of business. The original designer of this setup used .080 plexiglass on his and it came out looking great. He painted it with satin black lacquer paint before he set the gauges in and it looks like it was made that way at the factory. I wish that guy still had his website up. His user name was beavrab, but he's apparently cancelled his user name because I can't find any info on him.
If you want bigger gauges, you'll probably need to fabricate an entirely different template. This one is specifically made for Autometer Street Rod series gauges and is intended to replace the face of the sweep speedometer.
When you make the panel, you'll also have to fabricate some spacers to go between the back of the panel and the instrument housing. I used some small pieces of round plexiglass for Uncle's panel, and just epoxied them into place. When you take the speedo face off, you'll notice that the inside of the housing is uneven from where the speedometer mechanism used to rest, and will not let the panel sit level without the spacers. You can easily place the spacers so that they won't be visible when the panel is complete.
I wish this were my idea originally, but beavrab, wherever he's disappeared to, gets the credit for this one.