Can't find FURNACE filter

Question

We are trying to clean the filter on our Carrier Weather Maker 8000 horizontal furnace. The instruction book diagrams are very poor and marginally look like our furnace. By removing the blower door we see a filter in the middle of what should be the return box however it is not in a V configuration as shown in the manual and is not removable thru the door. The rest of the box exterior is sealed with heat tape with no apparent way in.any advice?

BW