Clamp Nos

Fitting New Air Filters for a '72 Honda CL350?

I got a '72 Honda CL350 a bit ago for a song. It needs new air filters. OEM filters are hard to come by and are out of my price range, so I got a pair of clamp-on NOS filters like these http://www.knfilters.com/search/product.aspx?Prod=RC-0412. However, the NOS filters are not the same shape as the original filters and don't fit in the air cleaner boxes (they're too long). The way that the air cleaner box fits together with OEM filters is that the back of the filter forms the back of the box, which won't happen with NOS filters.

How have other owners of vintage Scramblers solved this problem? Thanks for the help.
Edit: These are the correct NOS air filters for this bike. Original filters for this bike are no longer in production.

I would rather not cut holes in the filter boxes because I don't want to have excess water and contaminants getting to the filters--that's why the box is there.

As popular as this bike was there's got to be a vintage parts outfit that has an aftermarket filter made that does not fall under trade marks any longer.

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