Here are some pictures of the installation: http://imgur.com/a/UWSyD
This furnace is actually in my unit. It looks like the previous tenant or landlord just jammed a bunch of filters on both sides. I find it hard to believe that this is the right installation but can't really figure out how a filter should be properly installed? Has anyone seen anything like this? Or was the whole furnace just not installed right to begin with. Seems very weird. The 4th and 5th pictures are what is on the opposite side of the furnace.
Best Answer
Can't tell for sure, but it appears that the return air intake is on the "back" of the furnace.
You'll have to measure the opening, and then buy a return air grill that can accommodate a filter.
You may have to install a bit of ducting, or seal the grill to the furnace using some foil tape or mastic. You'll want the return air to all be pulled through the filter, so you want the grill sealed to the furnace cabinet.
Once it's installed, you'll just have to pop open the grill to replace the filter.