AC Guy,
Please, be careful on your "advice", as it can cause serious issues. This is our job...These fans are LARGE and Powerful, and help produce the high duty cycle of the welders. In fact any restriction is a grave concern. If you don't believe the power on these, ask Ram and he can verify as he found out the hard way how powerful they are. You will loose duty cycle. It would be better to periodically clean out the unit than to create other problems....It gets dusty and nasty in about every welding shop and the welders can take it as long as routing maintenance is followed. What is likely to happen is that while the restriction on the filter may not seem great to begin with, if forgotten or neglected, the restriction can grow as the filters start to slowly plug and un-noticeably plug.

Nikola,
IF you continue to keep this on the back, keep in mind you are taking your units health into your own hands, and you MUST maintain it weekly to keep good airflow. Also creating a box out of the filter, will increase the surface area around the back of the fan, and make "breathing" easier.