Spin casting a mirror would be an extremely difficult job to do. The epoxy doesn't want to cure to the same size everywhere for a start and this means that you will have to surface the mirror afterwords.
Next is that while the epoxy is curing, you have to maintain an excellent control of the spinning, a lot better than you would suspect. Even slight variations of spinning, on the order of a truck driving by on the road outside will affect the surface.
Epoxy mirrors that are done in the industry for optical work are all replication mirrors - a mold is amde (convex parabola or other desired surface) and the epoxy is put on that. Getting a 1/10th wave surface is difficult to do with mirrors of that design although extreme care taken can get there.
Good luck as doing so is an exercise in frustration by all that have tried it.