Molding a long, flat object?

I want to cast a long, thin part in hard urethane (80D) for prototyping. The part will be about 33" long, about 5" wide and about 1/2" thick and is a long radius arc of a circle, as though you have cut a sector from a disc.

It is intended that the upper and lower surfaces of the part will be rolling surfaces. The lower surface is supported atop some transfer ball casters and a second piece can be rolled along the upper surface. Therefore, I want the upper and lower surfaces to pretty flat and straight.

1) Is molding an appropriate method for what I want or should I be looking at a different process?

2) Would a silicone mold, with a support structure to maintain rigidity, be sufficient as a mold? How do you suggest maintaining straightness and flatness?

3) How do I fill the mold?

Do I create a two part mold, tilt it a bit and fill by injecting from the lowest point? Is there a way to fill the open mold and lay the second portion in place without making a giant mess? How would you actually make this part?

I do have vacuum equipment suitable for de-airing. Any tips relating to mold release, etc. would be appreciated

Thanks