Very interesting thread!
I have found the chart above to be very helpful. You can calculate the required filtration area from this chart in sq. in. and apply it to filter element(s) of any shape or configuration.
Bear in mind that this chart is based upon a 14" diameter round air cleaner fitted with a new (clean) filter element. On street driven applications, I generally size mine at 135% to 150% of equivalent sq. in. filtration area to allow for the gradual in service buildup of dirt trapped by the filter element.
My current project is a vintage car having a relatively low hood line and Weber carbs as do many of the Cobras here. Not wishing to modify the hood nor external appearance of this car...... I am building an air box enclosing the carbs, supplied via 2 ducts taking cool air from behind the grille, ahead of the radiator. Will install appropriately sized flat filter elements at the points of air entry.
My engine displaces 427 cu.in. and I have designed it to peak at relatively low redline of 6,000 RPMs. As you can see, the chart indicates a 14" dia. filter 3.5" in height having approximately 154 sq. in. filtration area. Allowing for decrease of effective flow capacity in service as the filter traps dirt...... 135% of 154 sq.in. would be 208 sq. in. and 150% would be 231 sq. in. So I will locate a couple of late model flat filter elements having a total area between them equivalent to between 208 and 231 sq. in.
FWIW...... A pair of square filter elements, each measuring 10.75" X 10.75" (or equivalent dimensions in rectangular or round shapes) will net 231 sq. in.
Hope this gives you some ideas.
Best regards,
Harry