I have a growing collection of 28mm figures based on 25mm mdf disks. The Spectre Miniatures figures are really nice but they can be a little fragile and easily damaged in the normal foam trays. No one likes a broken gun barrel.
Retrofitting magnets to the mdf bases is entirely possible, but a huge faff.
The EVA foam that is sold in Aldi/Lidl as flooring and used by cosplayers to make their props cuts on a laser cutter really nicely.
This tray is showing some 28mm metal miniatures being held really securely in a tray cut from EVA foam. Works a treat.
The MDF tray is my own design, 2 fit in a 4L really useful box and 4 fit in a 9 litre. Half widths work because my cutter only goes up to A4 size and for skirmish games I can store units together in one of these half boxes and then take the selections I need off the shelf and into RUBs for transport.