Any reason you don't want to use a responsive skin? It is the way to go. We have a very 'light weight' one in that the coding and features are nice, however it's not super heavy in load time.
https://www.bcsengineering.com/store/mobility-x-cart-responsive-template.html
Most of our clients use this one and prefer it over other options. Especially because the checkout is nice and streamlined too.
Carrie