Here's a filament spool that can be easily dismantled and assembled in a variety of sizes, without the use of glue or any plastic welding.

There are holes provided to secure the assembly with M4 or M3 bolts for the ends, and any small wedge/pin to keep the spars clamped into each end, but you should be able to keep this whole thing together with just a bit of string if you really want.

This spool is mainly designed to ride on my Mendel-top spool holder: http://www.thingiverse.com/thing:8290 However, it should work fine in any other setup with a horizontal axle. You should probably only use it on its side if you have glued/welded the segments together on the bottom.

• Pick a radius for the spool you're making.

• 120mm should be big enough for reels of plastic up to about 2kg or 4lb, while 150mm should handle heavier reels.

• Choose a spar for the width of your plastic reel.

• You can try finding this with a caliper around the sides of the reel or by holding a ruler up to it while holding the filament together. A little compression from the sides can be good to stop the reel getting tangled up.

• Print and assemble the ends of the spool.

• Use 608 bearings if you want the spool to have a ridiculously smooth turn, otherwise use a simple bushing.
• Some light cleaning up of parts might be necessary if you're using 3mm filament like me, though I have done my best to adjust the part tolerances so that it should fit together straight off the printer.

• Place your reel of filament over one assembled end, and slot the spanning members through holes inside it so that there is light pressure pushing the reel apart.

• Thread the inside end of the reel through something to secure it, you have all kinds of holes to pick from, so go with a nearby one, or even clamp it to part of the structure with a bulldog clip.

• Place the other end over the spars and secure the spool from either side with some kind of wedge.

• A bit of scrap card cut into sharp triangles and slotted through each end hole should do it.