Air Conditioner Filters

I have a Dometic/Duo-Therm rooftop, ducted A/C-heat pump. I get SO aggravated at these silly filters that you have to cut to fit inside the return vent! The thin, floppy filter is always getting sucked up into the unit, or is pinched in the grill and sticking out where I can see it, which isn't pretty. Has anyone found an alternative which will prevent unfiltered air from escaping into my coils. I don't mind washing it once a week. But I can never get it back in right when I'm done, so I put off cleaning it longer than I ought to. Any help or advice would be appreciated. My husband suggested cutting a piece of chicken wire and seeing if we could sandwich the filter between it and the grill. That will be my next try if no one has any better suggestions here.

THANKS!!!

Cheryl