Is the opening for the return air on the bottom? Can I open the side for a side return air application?

This return air box has an outlet hole in place at the top of the box, this is due to it being used for the up-flow applications. This hole measures 25" long and 21 1/2" wide. The box will also have the filter rack drawer that will pull out from the side of the box. We recommend that you give us a call if you have any further questions. Thanks!