I built a very similar box for my single cab. The only real difference is that I went a little taller, and modified the back panel to fit it. I use 3/4mdf to build mine, and have the woofers separated. I lost 3" of leg room from stock with the box, and I am 6'1" and am comfortable in it. My box hold a 12" and a 10".
My question is, is your amp visible through the back window? That's asking someone to steal it for sure!