Router sled rails, what to use?

I am about to glue up my SYS laminated top to my Moravian bench. I had planed to do a mostly hand tool build but after I have used a jointer and planer to get all the boards square I am wanting to get this done quickly so I can start using he bench. I will however attempt the legs and tray going full hand tool though. I am opting to make a router sled to flatten the top and bottom of the bench. I figure I can use the same material for the rails and sled construction.

Question: What do you think I should make the sled rails out of? It will need to be about seven feet long.
I can use MDF factory edges clamped to the sides of the bench with spacers to allow the bit full router bit travel.
I can also use plywood and rip the rails out of that.
I guess a third option is to use a 2×6 and joint one edge but I see some possible errors in dead straight.

Right now I am leaning toward MDF with three clamping points. I think I will get a monster level to help check flat and level.