I like the dry box design descended from tcosta's design but I don't like the way the spool holder has to be bent open and the spool slid in without snagging.   So I designed this one with an axle that can be dropped in vertically and  popped out by pulling up.  It does have the possible drawback of not allowing the dry box to be turned completely upside down (at least not if there is much filament on  the spool).  – - [After writing this I gave it a try, and it turns out I could turn it upside down.  Still not recommended though]

Its probably not obvious from  the photos that this uses an over-center design that grips the axle and requires force to push it in or pull it out.  This is the first of two designs and probably the better one. This one requires hardware in the form of specific screws.  The next design requires no hardware and is more fun but isn't as strong and is harder to print.

You will need:

8   M3x6mm Flat Head Machine Screws

4  m3 square nuts

4  m3x8 screws  (some latitude here)

ASA or ABS filament

The filament has two requirements. It shoud be low creep and glueable.  I used ASA and acetone slurry to glue the parts together.  Another reason for this filament is that a drop of glue should be used on the end of the  screws to secure them.  There is very little space to work with  so the specified screws are pretty much a requirement and there is no room for a nut or even a heat-set.  Speaking of which,  for parts like the frame I much prefer square nuts.  If you don't want to use them, hex nuts will work in a pinch and you could still use heat-set inserts perhaps with a little drilling

It is designed to grip an 8mm by 87mm rod which will serve for spool holders using 608 bearings.  It also works, less well, with 5/16 inch dowel rods etc.

The frame will fit the dry box bases mentioned above.  You need 2 frames printed in pretty much anything.  Print 2 knuckles and 2 mirrored knuckles in ASA or similar. Print 2 links with your choice of filament.  The parts can all be printed without supports except for the tips of the frames since they must be printed lying down.  The “knuckles” come in two parts which should be glued together for strength although they do work (for awhile at least) as separate parts.  Assembly should be obvious. Then carefully put a little glue on the end of the screws to keep them from backing out in use.

Finally, this type of spool holder can have problems with filament unwinding and coming off the spool.  The tensioner, printed in TPU, can snaps onto the frame when needed. It lightly rubs on the spool to provide slight resistance to the spool over spinning.