If I understand your question,the how is far easier than the doing when it comes to metal ducts.
If you have the tools and skill to do either of what you mentioned,you should just remove a section of return and replace it with the filter box.
If that is the return you show in red sketch and it is held to plenum with screws I would take that section to the tin benders and have them do it. Those pieces on each side of the vibration damper slide on and off to uncouple it.
I fear I am not understanding what you are talking about nor what I see.

ok, redid pic so it shows actual duct. i do not think i have enough room in overhead joist bay to move vertical duct sideways more than 2" or so. i need more than 2".

I assume the duct on left is the supply and the one on the right is the return. Is this correct?

Can you take a picture showing where the return connects to the furnace? This would be the best spot to install the filter box.

Please clearify wherther you plan to do the fabrication or tell somone else how to. As I mentioned origionly,if you have tools and skill for this work,laying it out should be a walk in the park for you. If you are thinking of instructing the fabricator,you are making a bad mistake doing it with information from 3rd parties. I'm sorry if I am mistaken but I sense you are intentionaly omitting something very relivent.

i see comments of adding 90degree filter box and so on in other threads. i watched a few utube videos of filter installs. seems they all go on the base/inlet of furnace. of course you than install the cold air return and run the rest of ducts. thats on a new install or new system install. must be harder with adding a filter box on an existing system. i can do whatever cutting or modifying of ducts is required.

It looks like you have enough room to install the filter box against the furnace. If you need a little more room you could move the return duct two inches to the right towards the utility sink. You can then make a transition back towards the left where the current noise isolator is installed.

Trion makes an Air Bear Cub assy. It's 5.25 cabinet with a 16x25x3 filter.

Looks like there is a filter slot just before entry to the furnace chamber; problem is that the filter is way to close to the 90ðell, thus, unless there are turn-vanes in the 90ðell, the bottom half of a filter, in that location, will have a lot more air velocity going through the bottom half of the filter.

This is typical of present day filter duct installs...not a good situation; plus most filter areas are way too small so the velocity through them is way higher than 300-fpm (manual D requirement) for a cheap 1" deep throwaway filter.

You said "i can do whatever cutting or modifying of ducts is required." So,anywhere between top of furnace and the ell,fabricate a tapered transition from present return size to 20x25 and connect to intake side of 20x25x1"-2"-3"or 4" filter box. On discharge side of filter box,fabricate a tapered transition from 20x25 to origional return size.

yes, pretty lame question. uh, my gap is 4" now. how do i make it 8"? well, you move the cold air return 4" to the right. too bad sink is in the way.

like this