Get some Velcro Strips and attach cover with that - easy to remove.

Why do you claim you need to remove cover to clean? If the cover was screwed on I doubt that you would unscrew it each time you cleaned it!

Don't make this harder than it is!

@Stax beat me to it. Just look for "velcro strips with adhesive". They are available everywhere, and come in different sizes. You can also get the velcro with adhesive as a roll and cut it in the size you want. Here is a listing for the 1x4 strips: 1x4 hook and loop strips with adhesive on Amazon

Good idea Stax, thanks. I don't know if I'll ever need to remove the cover to clean. When I googled toe kick vent covers a few videos mentioned cleaning the duct. Just trying to cover all bases.

EPA On Duct Cleaning

"Duct cleaning has never been shown to actually prevent health problems. Neither do studies conclusively demonstrate that particle (e.g., dust) levels in homes increase because of dirty air ducts. This is because much of the dirt in air ducts adheres to duct surfaces and does not necessarily enter the living space. It is important to keep in mind that dirty air ducts are only one of many possible sources of particles that are present in homes. Pollutants that enter the home both from outdoors and indoor activities such as cooking, cleaning, smoking, or just moving around can cause greater exposure to contaminants than dirty air ducts. Moreover, there is no evidence that a light amount of household dust or other particulate matter in air ducts poses any risk to your health."