If it is not square, as the one I purchased was, return for refund & seek other source.
Some furnaces may have an internal filter rack, or a location where the air filter sits that is accessible by removing one of the doors on the furnace. Most manufacturers recommend an external filter rack as it makes the routine maintenance much easier. The filter rack is designed to be installed between the side of the furnace and the return air duct, and usually requires some basic transitioning unique to each application. Changing your air filter regularly can help to avoid a lot of problems and ensures your furnace blower is not overworked.
The filter rack does not have a door cover, but is designed for the filter to slide-out of the 16 in. slot. As everyone's application can be slightly different, it may be best to contact a local sheep metal company to customize an external air filter rack for you.
This rack is made only for 1 in. Filters, as that is what is most popular residentially. You have options at the home center on several grades of 1 in. air filters that will fit this rack.