The 1/1200 scale ancient ships were temporarily re-based for Hail, Agrippa! simply by sticking the old individual bases to cardboard. In order to use the same ships for various game systems, they are being fitted with rare earth magnets. Custom 40 mm x 130 mm magnetic hardboard bases were made by Good Ground LLC. The magnetic material on the bases is intended for transport using steel drawers. Here they are being used upside down so the rare earth magnets on the ships will stick. This allows for variations in the type, number and positions of ships.
[Edit: Since the Good Ground bases are magnetic, a piece of ferrous metal on the bottom of the ship model works as well or better than a rare earth magnet. My recent ships have been built using a small zinc-plated steel washer or a square of tin-plated steel sheet. This is easier and cheaper than the magnets.]