Whoops, guess I missed your reply, sorry for being so late!
For the tubing connections, there's an easy way to secure them without having to weld.
Cut a circle out of aluminum, draw the diameter of the tubing you want to use over the circle (the circle needs to be 1/2" or bigger than the tubing).
Make some cuts radially out from the center and ending at the line you drew to make mounting tabs.
You want them to be anywhere from 1/4"-1/2" so they'll accept a rivet.
Cutting a circle out of the center with a hole saw or similar will make it easier to use shears for the tabs, and will result in fewer sharp edges.
Take these tabs and bend them out 90deg.
If you cut and bent it all correctly, the tubing that you want to use should fit snugly inside of this mounting flange.
Insert the tubing, drill a hole in each tab through the tubing, seal it up good with silicone, and rivet as the silicone is drying.
Rivet/seal the whole assembly to the box, using the 1/2" extra on the mounting flange to mate up with the box.
You can use variations on this idea to suit your purposes, like using nuts/bolts instead of rivets on the ring so you can remove it, using sticky foam to seal it, etc.
The picture below shows one way of doing it, but if you want the airbox to look nice on the outside without visible tabs and rivets, mount it backwards so what you see in the picture is on the inside of the airbox.
Push the intake flange out of the airbox until you get the right length, rivet, then cut the excess tubing from the other side, flush with the tabs.
