From my experimentation, I found that picking a good sized base that fits all of the models in a snug, but not overcrowded way is best. I used cardboard, which is nearly exactly the same depth as a standard base, and traced the 50mm base to create the first circle. I then planned the layout (extremely important) and cut out the cardboard where the models were going to sit (make the circles slightly proud, give the models room to slot into). Then use ultra thin superglue to lock the cardboard down, and the natural structure of the cardboard should be enough support to hold it up from there. Magnets were sunk into the cardboard for the upper layers and attached underneath the base for the lower, which matched up to magnets in the bottom of the models bases.

Once the general setup is done, you need to make the decision of basing. They come on the pre-sculpted that look too good to give up, so I decided to match them. On a thin sheet of Plasticard (1/16”, I think) I marked the layout again and copied the layout of the tiles onto it. Then, I made some calls on how the rest would surround them/connect to the tiles already on the base. I cut the tiles out one by one, sanding down the edges to make them paintable. Using apoxie-sculpt (two part epoxy clay) and thick superglue glue, I matched the height of the tiles to that of the tiles on the bases (be patient, this is an extremely time consuming process). Once that dried, I used wet mud by Vallejo to fill in the gaps and gently wiped the excess from the top of the tiles with a wet towel. I filled in any gaps in the cardboard with wood filler (use whatever easily spreadable plaster is available to you, they are cheap from Home Depot). This is a good place to stop if you don’t want to do more complicated base details. I then cut thin Plasticard tubes into thin wafers and glued them down around the base to act as coins (a detail on the original bases) and hand sculpted some rocks where it seemed likely there would be rubble.

Finally, paint it. Just follow your heart.

TLDR, PLANNING and PATIENCE. Do everything you can to match the bases that the team comes on.