Need to move my vertical HVAC return approx 3 inches away from my furnace

I installed a power humidifier to my furnace side (had to be mounted facing the face of the A-coil on the plenum per OEM) . and it’s mounted in between my furnace and cold air return.

The unit (Aprilaire 700) needs about 3 more inches of clearance to take off the cover and do service like changing the water screen element. I have 10 inches, need 13 inches.
It appears I just need to put in an offset but am not clear if I can buy off shelf .

I did some research on the "OWL method" (Offset x Width / Length) and although this seems slick as an approach in practice it requires your offset piece to be cut at angles provided in the formula . My duct is 10: x 20" square. Taking a duct piece if you snip each end at an formula angle, unknowingly you are making the mouth of the piece larger than the perimeter of 10 x 20" . That being said you will not be able to fit it between duct run.

The other option is to move the whole vertical run and forget the offset. Problem here is at the bottom it curves into my Spaceguard filter case. I would have to get a 3 inch deep extension which I’m not certain about. Ideally I thought I could just splice in a 3 inch offset in the vertical return when I viewed the You tube video.