The silica-gel I am using can be used to monitor the moisture saturation level and has the added advantage that after drying it in the oven, it can be re-used indefinetly. Once the air is vacuumed out, the four spools in the bag become one big pack, which is easily stackable if necessary.

I have written a short article about it, for those which are interested in a cheap and easy dry filament storage solution. Just check it out on my website: 3dprintingforbeginners.com
EDIT:[I would have liked to place a direct link, but am not allowed to do so - sorry for that].

Feel free to let me know what you think about it or share your experiences with that setup.

Those cartridges are too cool.  I would make sure he did not discard them if he was my friend.  I'm sure you could retrofit them in some way.  I would need to see the inner workings a little better.  Very cool though.

For opened box filament, I keep all of them in an electric dehumidifier to dry them up even more.  I typically dry them up to around 10% - 15% humidity.

I know many of us don't have an electric dehumidifier.
You can try this instead.
Preparation:
a container with lid that is able to seal tight (water proof)
package of dehumidifier (also known as desiccant)
Keep all opened filament inside the container along with the desiccant.  This will keep them around 35% humidity level depending on how long and how many desiccant to keep inside.  But 35% is an acceptable level.

Give it a try

Notice the Desiccant packs within the corner voids; the rubber gasket around the spool area to seal off the entire chamber including a grommet where the filament exits; and the dual rollers to guide the filament...so you get great storage and just plug it in when ready.

It's strange that Stratasys neither credits nor pays for shipping to return the used cartridges.  My buddy says it's just not worth it to him to ship the empties back.  A shame!  I suppose I could just rewind and re-use these as opposed to discarding them.  Any other ideas?

For now, I have a damp-rid in there with them..

What about the vacuum bags for clothes?

Type gamma into their search.

You will see the lids now also under the leaktite brand

Check your local store inventory or order online.

5gallon buckets with the screw top gamma seal lids and dessicant, works great for me.

where do ya find the ones with screw on lids? do lids also have rubber seal?