4 inches is not 100mm so my filter lets stuff past it

I have been struggling with algae getting past my filter and blowing back in to my pool. I tried many different PA90 style filter cartridges and they all did the same thing. I then measured the opening of the cartridge... a perfect 4 inches. And the opening in the filter housing was a perfect 100mm. There is about a 16th of an inch difference so of course stuff is getting through. Has anybody else solved this problem.

Hayward C900 Star Clear Plus