Alright, found a 3D model of the Ikea 365+ 10.6L container that everyone uses as filament box. However, it did not have a modeled lid, wich was exactly what I wanted.

My measurements are close, BUT not super precise, so don't complaint when I share the model, when reality is you should NEVER design something tightfitting for plastic containers like this, without a real life reference.

Now that I have the lid, I can model what I want to do to turn this into a rael filament box.

Alright, modeled a pair of 1Kg spools from my go-to filament brand and set them up inside the container, as high as possible (so there is space for the dessecant at the bottom) and to the rear, so the filament can have a softer path to go to the outlet (wherever I put it, still undecided).

Things I know I DON'T want:

Drilling on the container body. Ever tried drilling those things? The plastic will shatter like crazy and drills will grab and pull like a velociraptor, and that's made even worse because the sides are at an angle, and drilling both the same is a pain in the ass.

#3DPrinting

Loose things inside the box (apart from the dissecant).

Roller carriers that are just sitting inside and can move and wobble around are just a no-no for me. It's just a matter of personal preference, nothing particularly wrong with it per se.

Plan is as follows.

Attach everything to the lid. It's way cheaper than the container, AND it's flat, so much easier to hold and drill, win-win.

Will make aluminium holders for each spool, and attach them (with screws or rivets, unsure about it) and use the springiness to hold the guide bars.

The guide bars have flats to keep them steady (and let the bearings to the work) and are staggered (same piece with the top bent one side or the other) so they can't actually conflict when putting new ones in (the steps should NOT be thicker than the alu anyways, but just in case...
Alright, after assembling a test cover, the latching mechanism for the spools that was so awesome in my head, is pretty crap (the cover just gives way too easily, so the springiness is inconsistent. So I'm going to mod the pieces to have a U shape for the axles to catch and hold well.
Proper fancy, because reasons.
Looking slick, imo.
Pretty happy with it, now onto designing the actual roller-axles whatever, to fit in there.

Done!

Will cut the pieces tomorrow and assemble on the weekend-ish.

I wonder if it will hold to riveting (with the large rivet face to the plastic)

@Nixie are you going to use flat washers on your rivets? Just a thought.
@kingwookie the bottom is aluminium, should hold, but I want to try washerless on the plastic, out curiosity, first.
@kingwookie tested the riveting, I don't feel there is a need for washers on the plastic side, as long as it's the rivet head against it.