cabinet bracing

i have a juwel vision 260 aquarium and cabinet. i have removed all internal supports of the stand and have created internal supports for the top of the tank. due to the size off the sump it fills most of the cabinet due to the supports i am looking at removing the supports and using stainless steel box sections supports, the sump overhangs the back of the cabinet so i cant box of the rear of the cabinet to help strengthen it.

heres my question

what size stainless would i need, best method of construction as would like to maximise cupboard space