if you already have a side panel and just want to add a fan you could just add a spot for one with a drill and some tin snips
get the fan size you want, make a template of the bolts/fan size mark spots on case with a marker then drill for mounting hardware and a pilot hols to snip the hole into or if you have acess to a dremel that'd make it even easier with a multipurpose cutting bit, chews through aluminum with ease