On Fri, 17 Jul 2020 19:44:37 -0700 (PDT), Jay Pique
wrote:

On Friday, July 17, 2020 at 12:32:08 PM UTC-4, wrote:
I'm looking to cut 3 pass-through mortises (2"x6" long) on each of eight large pressure-treated square timbers (8"×8" wide x 8 feet long) for a small post & rail fencing project.
I've cut mortises on smaller thickness using just hand tools (drill, chise, wood file). I can't justify buying a plunger router or drill press just for this project and hardware companies near me don't rent them out. My plan is to purchase a longer drill bit, drill numerous holes along the inside edges, chisel out an opening
on each side, use a sawsall tool to cut around the perimeter, and use an oscillating tool to clean up the edges. This will take some time since I have 24 mortises to cut. Not needing a tight fit since the rails will be loosely placed inside the mortises. Any other suggestions on how to do this easily with hand power tools?

So you've got drills, bits, chisels, sawzall and an "osciallating tool"(?) but not a router? Huh. Then mark your mortises, drill them out from each side, and chisel them rectangular until they hopefully meet square in the middle. What is it you're really looking for here, though?-like a miracle "how to turn your cordless drill
into an 8" slot mortiser"? smh

You could almost do it with the oscillating tool. Have to adjust the
design so that the mortise is wide enough to take the tool head is
all.

And 4 inches is a pretty deep cut for a router.