Product review (4 stars): Can't be a wimp, work at it a bit and it will be fine... :)
Used this in a custom size as additional filtering for a range hood.  The filter is in 3 layers, mesh screen (grease trap), carbon filter and expanded metal (for support I imagine).  Cut this to size and gotta admit it was a bit of a struggle to assemble the side rails.  Wasn't willing to give up (kind of stubborn that way) and after cutting the rails at 45 deg at the corners, pressing the little corner tabs in 1 by 1 and working around each edge, it took about 15 minutes to complete.  Used a flat blade screwdriver to push all the loose edges in until it was clean.  A little tricky but if you need a custom size it's a fair product.