I set two benches end to end, then built a table top on that and leveled it. Beam on top of that and shimmed it to as close to level as possible down the center of the beam. Built a simple base for the router (with a fresh bit) to slide around on, and attached a large base to the router. The base has to have flat square surfaces so the router is moving on a flat level base. I Clamp the base to the beam, and then move the router back and forth on the Base. Move the base down the beam, rinse and repeat. Seems to be working really well. I coated the surfaces of the base and router base with Butcher's wax so the router slides easy.
I'll tune up the surface with a power planner afterward. I tried just using the planner to get a flat surface, vs a router, but it would have taken forrrrever. Once I got the process down, I did half the 12 foot beam in about 45 minutes.
