Memorializing a note that when wargaming with Robert, we have standarized on 60mm frontages for bases. This works well with almost all rules for figures at the 25mm, 28mm, and 1/72 scale. 15mm scale would then use 40mm bases.

See a discussion here on 60mm and potentially using 25mm or 1/72 plastic figures as the scale for these frontages.

Here were bases Stan bought but that weren’t right scale.

Stan’s Litko Bases (not the right size, but shows example components).

To actually have the 28mm figure be attachable, here’s what Litko said:

Most customers use the magnetic base bottoms on the actual figure bases, then use the flex-steel base bottoms as inserts in the trays.

Your half way there, you can use the base bottoms you have and stick them into the tray. You just need to order the magnet base bottoms for your figures. Looks like your using the 25mm circle bases.

http://www.litko.net/products.php?product=Circular-Miniature-Bases%3A-25-mm

See pics below for Robert’s figures.