I have tried many different storage solutions over the years and I believe I've finally found the best one:
https://rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.co.uk%2Fulk%2Fitm%2F391763898671
It's £17.49 for the larger box, + £3.95 delivery in the
UK.
It has a top tray with a handle which is big enough for infantry, bikes, and even redemptor dreadnoughts. Remove the tray and underneath is a cavernous compartment tall enough for a Hemlock Wraithfighter ON its flight stand to sit comfortably.
The best part is that it has a sturdy handle, allowing you to carry all this in one hand with ease. The lid doesn't 'click shut' but unless you're tipping it upside down it doesn't matter.
All you have to do then is purchase some adhesive-backed steel paper, and to superglue neodymium magnets under the model's bases (2mm think + 6mm diameter should do the trick) and carefully line the tray and box with the steel paper.
The SMALLER box even fits my entire Custodes army in it, including 2 Caladius, 4 Dreadnoughts, 10+ bikes and 25+ infantry. I just had to get creative with creating more internal storage by adding a removable shelf and keeping the Caladius' turrets magnetised and detached when stored.
It may be a bit more work magnetising everything but I think it's more cost effective than loads of bespoke foam, and once it's done it's waaay more convenient.