I have a Leigh FMT Pro jig, and once borrowed use of my neighbor's deck railing. I clamped the FMT backwards to the railing so the long board could hang down past the edge of the deck. I have also used the edge of my shop attic pull-down stairs for shorter work that was still too long to mount the FMT to the bench.
Without an FMT jig, I would probably do the same for positioning and clamping my long work, but would use a template and guide bushing with the router. I've also come to appreciate "Floating Tenons" when using templates, guide bushings, and a router, since you can get great results using the same template to cut both matching mortises. Making floating tenon stock is easy with a table saw and then a planer to get the desired thickness, but it can be done with just the table saw. Make pieces the desired thickness, and then cut tenons from it as needed, keeping in mind that the tenon needs to be about 1/8" shorter than 2X the tenon depth. The 1/2 round ends of the routered mortises can be left open. It isn't necessary to round the tenons to fit. Leave them with square ends long enough to fit the flat sides of the mortise. The 1/2 round end spaces will take the excess glue. It's the flat side surfaces of the mortise and tenon that provide the joint strength. They should slide together when being dry fit, with enough friction to keep them from falling apart, but should not require pounding with fist or hammer to dry fit them together. When sized this way and the glue is added, they will be very strong.
Central North Carolina