This is a 100% printable filament spool based heavily upon randyy's design.
Added webbing to arms for strength.
Modified dovetails for better fit (the original required a lot of sanding to get to fit well - for me).
Increased arm thickness for strength.
Increased mount size for strength and more solid connection to MakerBot.
Changed geometry of birdmouth slightly.
Increased size of filament holder so that more filament could be wound on spool.
Added hole in the filament holder.
Beefed up the filament clip on the filament holder.
Enlarged the conical pieces that press fit into the arm for more secure fit.
Added a filament guide for use with Cupcakes.
Print out and assemble. The dovetail parts should not need any trimming except for part that lies against the build platform. Even so, if you assemble it when it is warm you may not even need to do that.
Wind the filament onto the spool. Start by threading the filament through the holes on the "U" shaped parts. Wind the filament so that it just lies gently in place. When you have finished loading the spool, cut the filament and then press the end of the filament into the built-in clips to keep it from unraveling.
Mount the filament guide onto the cupcake just above the middle bolt as is shown in the picture. The clearance between the spool and the guide should be enough that you are sure that the spool won't hit it as it turns.
Sketchup file is included.