I haven't made a "cage", but I've made a few roll bars to help protect the engine and block direct impacts with the rear shock tower. I used 5/16" cold rolled steel bar.
With each one I make, I make them a bit better and apply more thought to it. I should make a new one for my aftershock... but I guess I don't care.
Savage old (first one I ever did on a savage I no longer own):
Revo old (second set I did on revo I no longer own):
Aftershock (3rd one I did, still own and use):
Extended HB Chassis Savage (4th I did and still own and use a lot!):
As you can see... I ran it before making the roll bar... stupid skate park!
Similar rear mounting plate to the aftershock one I did:
Sorry for all the photo's. Hopefully they help guide you on your quest to a solid cage!