Here's one I built for a mates 413.
It's built out of exhaust bends that we cut to the correct angles and weld together. The rubber bends thru the guard are from a Nissan Skyline inter cooler.



You can use PVC if you don't have access to a welder but I like steel as it takes a beating and doesn't deteriorate in the sun like PVC.
The lid on your air box has a neoprene seal which is probably stuffed so take it out and replace it with some silicone O-ring rubber to make sure you 100% water tight and as Mark said fill the drain hole in it, I used hot melt glue for that.
HTH Shane