I wouldn't bother with welding it, just use pop rivets (or solid rivets if you have the tools and skill).
.032"-.040" 2024-T3 would be a good metal to use, it's alclad (alloy that's clad in pure aluminum for better corrosion resistance), and it's been proven sturdy enough for airboxes in thousands of aircraft flying today.
You'd need three pieces for the box, one U-shaped piece that makes two sides and the bottom, two flat plates for the sides, and another flat plate for the lid.
Factor in the intake flange, drain hole, screws and rivets, and you're looking at spending maybe $50.
Sheetmetal:
http://www.aircraftspruce.com/catalog/mepages/alumsheet_2024t3.php
Pop rivets and gun can be bought anywhere (like Ace hardware), these would be your strongest (and most expensive) option though:
http://www.aircraftspruce.com/catalog/hapages/cherrymax2.php?clickkey=10160
Tinnerman nuts:
http://www.aircraftspruce.com/catalog/hapages/tinnerman2.php?clickkey=5485
Simple button head sheetmetal screws:
http://www.aircraftspruce.com/catalog/cspages/screws.php?clickkey=11590

Making the box itself is really simple.
Just imagine making a carboard box and using staples to hold it together.
You'll need to bend a small portion of the sheetmetal (about an inch) so the rivets will have a joint to hold.

-Cut and bend the sheetmetal (aviation snips for cutting, bend with a large vise or buy a sheetmetal brake at harbor freight for $20)
-Coat joints in silicon
-Drill two holes 1/4" from the corner joints and rivet pieces together (think of it like tack welding)
-Fill in the gaps with rivets spaced out an inch or so (more rivets per inch=stronger joint)
-Use self tapping sheetmetal screws and tinnerman nuts to hold the airbox lid on

You'll have to make a few right angle tabs to rivet the intake flange to the airbox (4 tabs or more), and seal with silicon.
I suppose JB weld or epoxy would work just as well as silicon too, as long as you get a good seal out of it.

If I had the materials handy and knew the measurements, I could probably make one of these boxes in less than an hour...bending and riveting is really simple.

End result is an airbox that's extremely lightweight and durable enough to last on aircraft for 10,000+ hours, no need for custom welding (or the costs that go with it), easily patched if broken, looks great (alclad takes very well to polishing btw), and the satisfaction of having built it yourself.


This should give you an idea of the joints I mentioned that you need to bend:





It's a great example of how simple an airbox can be made, all you're looking at is 3 pieces of bent sheetmetal.
There's some tabs bent over to match the other pieces so you can drill through and rivet.
They've already drilled in the right spots to tack the box together, once you get those corners riveted you can continue to add rivets as needed along the sides.

It helps to use vise grips where you drill and rivet so the pieces are straight and flush when the rivet expands.
One thing to mention that a lot of guys overlook when building out of sheetmetal...radius the corners and smooth the edges!
All it takes is a few seconds on the grinder/dremel or a few swipes with a file and those sharp corners won't cut anything.
A quick pass with a file along the edges followed by 80 grit sandpaper is all it takes to smooth out sharp edges along a cut.


If you decide to give it a try, I'll be around for advice...