I once built a Stinger cage for Tamiya's M998 using plastruct L shaped beams. It was my first attempt at scratch building anything (I still have it - but I'm not showing it off). I didn't have the actual measurements of the cage when I started (big mistake!!) so, I scaled down my pictures to match that of the M998 model and went from there. In the end it turned out very close to scale - I got lucky I think!
So, go back and take some really good measurements of your equipment and then figure out what it works out to the scale you are building in and go from there.
Allow for mistakes and take your time. Measure twice and cut once! Good Luck!