Hi guys, I am thinking to build my own head cab for OTS50. But since I want it to be as light as possible, I plan to use 15mm plywood (0.59"). I know lot of people use 3/4". What d you guys think if I use 15mm? Is it sill looking good?

I did that for another head.  Its OK, but you can round the edges and corners better with 3/4 inch stock.   Have you considered 3/4 pine ?

I made one head cab of pine and it worked out fine.  I only used the thin ply because I wanted to use up some stuff I had lying around.   I wouldn't do it again, though.