Roofish, as the years have gone by & I've accumulated more & more flies, I've obtained several different types of boxes. Used to be I stuck with the Plano type plastic boxes, and they work fine for some types of flies, but not as well for others. I've had some aluminum boxes, but have come to prefer plastic. Aluminum corrodes, while plastic does not. Plastic however cracks, so none are perfect. I'm also cheap, I can't see paying $20 or more for a fly box, when a similar one with a $6 -$8 price tag is available, particularly since some of the higher priced boxes appear to be identical to the less expensive versions.
IMO, a variety of sizes is a good idea if you tie or buy a variety of fly types & sizes. I've got flies of all shapes & sizes, and one size box doesn't fit all flies.
I like simple boxes too, waterproof is good, but not always a necessity. I have several boxes that have the slit, flat foam in them a few with ripple foam. All work fine IMO, and I understand none will last forever. I'm not going to last forever either, and expect most of the boxes will still outlast me.
Here's a few I have & like:
Plano, small size about 4"X7". Holds a good number of flies.
Fox Box, has molded plastic slots, seems to work better with heavier wire hooks, like those pictured. (saltwater)
Flambeau Double sided box. One side has ripple foam, the other is like the Plano box.
Large Plano, with carry handle. This is one of my boat boxes I like for big flies.
Not sure these are still made. I have two & they have slots that allow them to be attached together. Holds a lot of flies, even big ones!
Hinged & waterproof box. Got this one from an Ebay seller. Nice box IMO. has the pre slotted flat foam inserts.
Small pocket size Flambeau box. I've had this one for many years. Ripple foam insert, one sided. Only holds a few flies, but a good one to keep things light.
Large Flambeau polystyrene box. I bought several of these when I was tying commercially to use for selling flies of various sizes at shows. Not a box I would want to carry afield, as the plastic cracks too easily, but a good one for storage since the plastic is clear.
Large Plano, holds a lot of flies. Another good for storage, but not something I would want to carry.