Originally Posted by Scrapper
X2 what SeaDog said. Looking through the louver you can see the furnace. If you block the louver your furnace will overheat without air coming out the heat registers and you could cause damage to the furnace.
The installation manual for the Suburban SF-series furnaces specifies the required area for the return air opening. For all but the largest ones, I believe it is 55 square inches of open, unrestricted area.
On our small SOB trailer the furnace was very noisy and there was no place to get away from it. But it had 110 square inches of return area. I figured that was enough area that it could withstand the restriction of those older-type filters that just contain plastic strands. The area was simply two cabinet faces covered with grilles. So I converted the cabinet faces into filter holders and set the filters flush with the cabinet face. The grilles mount over them.
So far so good. No evidence of overheating or having the limit switch cut out.
If you are going to try this, look up the return air area spec for your furnace model first and make sure you have much more area than that.