sewage injection pump seal

I have a Water Ace sewer injection pump for pumping waste from my washing machine and utility sink in my basement. It is an above ground unit housed in a plastic bin. On the top there is a small access hatch to pull/clean the filter. When there is high pressure water coming in (from the washing machine) a very small amount of water seeps out (about the size of a quarter puddle on top). It's done this since I just cleaned out the filter. I just moved in and this is the first time I've had to clean the filter. Is there something I should use to create a better seal next time? Remember it has to allow me to remove the cover to clean again in a month or two. Thanks for advice!!

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