I machined the main body of the adapter from a solid piece of aluminum. The mounting ring was sliced off the first piece after drilling the four holes. Then it was divided in two so as to fit inside the airbox plenum. Both pieces were lightly coated with Hondabond #4. The sketch (exaggerated) on the right records the dimensions of the K&N air filter element I settled on. It's one of their generic filters, #RF-9160. It fits comfortably between the battery and the electrical panel. A slightly larger filter tried earlier was unsuitably oversize (it rubbed on the wiring). I prefer this use of the K&N product over fitting four individual filters.

These pix are not of my bike. These two pictures are of the two commercially available adapters I found online after I had made mine. I would guess they're 3-D printed. The one on the left came up in a Google search. The site is www.shapeways.com. They make plastic parts to order. The one on the right was found on a German site that called itself Classic Bike Parts. Here is the link to the modification. Neither looks as good as the one I made. But they might be reasonable alternatives to making your own in most cases.