A steel frame like here ?:http://www.thingiverse.com/thing:2156369/#comment-1257765
BTW, plenty of decent Prusa with a metallic frame. Smart people start with that or better design, not acrylic crap.
You would have been better inspired to do a correct Prusa with a metallic frame, not just a frame while keeping all the design shortcomings and errors of the Prusa.