Primary best practice is to provide good support for the top such that only small areas are left with nothing under them. It almost doesn't matter what the top construction is in that respect and within reason. I never had a problem with sagging on the two router tables I built...both used a double layer of MDF surfaced both sides with plastic laminate. The cabinet, particularly on the second one, left maybe only 12" "exposed" directly over the router compartment.

The same principles would hold for a table saw extension if built "traditionally". A "foot" directly under any dust control cabinet would provide additional anti-sag support, too. You could do a modified torsion box with a few inches "open" on the bottom right at the saw for easier mounting to cope with the issue you describe with fasteners.