They make acrylic filter sock holders that slide right in, or if youre handy you can make one out of scrap acrylic. Hard part is making the hole.
If you are going with miracle mud in the second chamber you definitely need to reverse your bubble trap baffles, you cant have water exiting the bottom of the fuge if there is substrate there, you'll have a huge mud mess. If you want the fuge in the front you need another baffle or something to break the water or your miracle mud will be flying all over the place.
I honestly don't think it matters if the fuge is in the first chamber or second. IMO nutrients are throughout the tank, they dont just all of a sudden drastically decrease after the skimmer. Because of this I would have the first chamber as the skimmer section and you don't have to worry about the turbulence of the water entering the sump and also saving space. You can also control (as shown in the first pic) the height at which the water enters the fuge (~6" above the bottom).
All kinds of ways to build a fuge, just my 2 cents.