I wouldn't recommend a lid right on the soap. You would never get it off. I use box that I cut to fit over my slab mold but I don't pour to the top. If you pour your soap at a thin trace you can get a really smooth top. I do anyway.
I use parchment paper to line my molds and the soap doesn't stick to it. If the poster used it between the soap and mold the lid shouldn't stick.
It the it likely won't be smooth either. I tried plastic wrap and parchment once when I first started and go wrinkles.
This is exactly what I am going for also! And, I just treated myself to a planer!I smooth out the batter with my hand once the parchment is on and before I put it in the oven (I cpop most to force a gel). Not perfectly smooth but very close ... Just a bit of planing after and it is good.