I have a 2x2x4 (LxWxH) and it fits perfectly on top. I like the 24" hood because I get an even dispersment of UVB/Grow light.
Seeing your cage is pretty large, assuming its 36" wide I'd get a 36" hood.
You can always message Todd, who owns LightYourReptiles dot com. He's a member on this forum aswell as a site sponsor. His username is Venutus1. You can PM him with lighting questions and he can give you great options and recommendations. He can even build you a lighting kit for your enclosure and get you started.
I highly recommend Arcadia lights with a grow light. Arcadia lights bring out colors on your Cham, cage looks allot brighter and balanced too. Grow lights are a must if you want to keep your plants alive if your enclosures inside.
-Gabe