As noted in the scenario:
25mm round MDF bases, sides slanted with a sharp scalpel (yes, an offering to the Blood God has been made) and a small blob of milliput and a round bead to push in said blob to make the crater. Some glue, some sand, some drying time. A bit of rubbing to remove loose sand, some sanding on the edges and these are ready for primer.
I'll add another dozen or so of the big 50mm one. As soon as I remember where I put the bloody bag.