I used cork tiles 4 mm thick and tore off some pieces. I positioned them on the movement tray (that I ordered online) and just started cutting . I fixed the cork to the bases and tray with PVA glue and made sure there was plenty of it covering the surface. This gives the whole a smooth layer to paint on. The bubbles were made by making little GreenStuff balls of varying size and pusing them into the wet PVA. The PVA glue is thick enough to embed the bubble. I will provide more WIP shots with the next units and bases so it becomes more clear. The tray has been painted and I have started to attach the models to the bases: I will provide pictures of the completed base + models tomorrow Thanks for all the comments and feedback, it is greatly appreciated!