Look in Roni's 300G build thread for a drawing of the frame that I used on his 270G tank. To be very honest, most of the home-built stands are *way* *WAY* (yes I meant to type that twice) overbuilt and the one I built for Roni is no exception. In fact, a 150G stand could be safely built with plywood sides and back and hardwood facing and no internal frame at all! (If you know what you are doing and your joints are well constructed...)
Other than straight edges and accurate cuts, the concerns you need to address in your design are load bearing capacity, shearing and twisting. The vertical supports in the stand (whether we are talking about an internal frame or the external face frame/panels) handle all of the weight. Although a single 2x4 stud can support well over 500lbs of weight (assuming no twisting or flexing), I still support corners of stands for very large tanks using two 2x's glued and screwed to form a 90 degree corner, more for stability than for weight bearing capacity. The horizontal supports at the top of the stand provide some support, but most of the weight is actually on the vertical supports in the corners and center of the stand. I usually use single 2x4's for the top horizontal supports unless I am spanning a large area with no vertical supports in the center.
Once you add hardwood or plywood facing to the frame, and I usually attach that with a combination of wood glue and screws, the stand will not be able to twist or lean, making it even stronger.