My brother built a 2x12 guitar cab last year and finished it with truck bed liner paint rolled on with a texture roller supplied in the kit. This is messy, wear rubber gloves. The results are well worth it, a durable finish that from a few feet away looks very similar to Tolex vinyl covering. I will try to attach a photo of his cab with a Crate amp head (Tolex covering) so you can see this.

He just finished another guitar cab for himself (1x12, 2x12) and a 1x15 bass gt cab for his son. He used the kit they sell at Walmart, not sure if it is Herculiner or another brand. You might also consider just checking with a commercial truck bed place (Gator, Rhino, etc) and seeing what they would charge. A local truck bed outfit here would charge, for a pair of large PA speakers, about the same as one of the "kits". I asked about adhesion to ply, and they replied even better than to metal. They have done the entire speaker inventory of a large commercial sound company.

I'll put one photo in the image link, which should show up in this post. I'll put a url link to the other photo.

The Crate head on top of the stack is Tolex. Compare appearance to the speakers.

• http://i15.photobucket.com/albums/a384/PaulCoats/Morris1x12Cab.jpg