I don't see the value in using a diagonal mattenfilter. You would get very close to the same filtering area if the filter ran the length of the tank, and it could be offset to one side to allow plenty of room for other equipment on either side of the filter foam. Using 3 pore sizes in series would certainly increase the polishing of the water, but my ordinary mattenfilter (in the tank) does a very good job keeping the water clear anyway, so it may not be necessary to use multiple pore size foams.
Mattenfilters are supposed to work best if the flow velocity through the pad is within a certain range, so a too big filter would drop the velocity too low for optimum biofiltration. You would want to check that part out first.
I would use something across the bottom of the tank to keep the foam pad in place, since you wouldn't have a substrate to serve that function.