Redesigned @leefla's model https://www.thingiverse.com/thing:3069436 to be a little bit taller as the filament rolls were touching the desiccant in the container with the amount I had in them. I also made the holes slightly bigger so the rods slid in a bit easier. I remixed this for the 3/8" (9.5mm) diameter aluminum or steel rod only. You may need to slightly scale the parts depending on how much they shrink and the diameter of the rods. I bought 4 rods from Menards and 3 of them fit perfectly but one was a little loose so keep that in mind and maybe measure or eyeball the rods before buying them to make sure they are all close in size. You can add a small strip of electrical tape to the end of the rod before sliding it in the part if it's a bit loose. You can also chamfer the ends of the rods so that they slide into the parts easier if needed. Each container will hold 4 standard 1kg sized rolls of filament and (for me, at least) stay around 10%-15% humidity.

You Need:
2x This part
2x Aluminum or steel rods 3/8" (9.5mm) diameter cut to 12 3/4" (325mm) length
20qt Sterilite Container (Available at Walmart or Target, depending on your location)
Desiccant beads (I recommend orange indicating beads that can be reactivated)

Optional: Digital Hygrometer (I recommend the ThermoPro TP49 on Amazon). I just set it on top of the desiccant in the corner of the box to keep track of the humidity. I plan on eventually mounting it higher in the box