5000 series will be ideal for an airbox. It is widely used in commercial applications, any propeller plate or checker plate you see on tradies utes/toolboxes etc are 5000 series.
6061 is structural because it responds well to heat treatment to achieve higher strength (but only if it is done properley, and only after all the welding and cold working is carried out).
Since most airfilter cases are made from plastic as they are not load bearing, a non structural grade of Alloy will be fine.
Consider the following though;
If you want better abrasion resistance (5000/6000 is as soft as butter in the annealed condition and easily scratched and marked) anodise the finished product. You can give it the brushed finish or polished as you prefer and anodise to preserve the appearance.
If you are concerned with the box fatigueing through vibration and developing cracks you could use rubber grommets to act as vibration isolation mountings. Good design and workmanship will most likely negate the liklihood of in service fatigue.
Anodising is also an excellent surface protection and will protect all surfaces both inner and outer from corrosion, plus a colour can be added before the sealing process to customise the finish if you like.
Be aware that there are better combinations of filler wire and alloy compositions to avoid welds and parent metal being a different shade when anodised. (silicon is used in Al filler wire to aid flow in the welding process, but the silicon has the tendency to anodise darker than the parent material which has a lower silicon content) Choose a Tig filer wire with a lower silicon content if anodising the finished product and aesthetics are important.
I have made air boxes of both welded and riveted ( solid rivets - wet installed )construction from 0.060 inch and no anodised finish which have maintained thier appearance and not cracked or split or corroded to any great extent.
You can always use scotchbrite to achive a nice even brushed finish that is easily "touched up" with a light blend and a rub with scotchbrite in areas that tend to get surface attack from abrasion and debris pitting.
Hope this gives you some options to consider.
