Update: If you mount your spools horizontally on top, I'd recommend using this filament guide instead of using spool guards: thingiverse.com/thing:9604

These are OpenSCAD Parametric MakerBot Filament Spool Housings. I always found Pax's excellent Spool Housings to be very difficult to print and kept thinking if only it were a little thicker so they don't delaminate or a slightly different shape, it could be even better, unfortunately I don't have Solidworks... so after having a few disasters with my stackable spool thingiverse.com/thing:9198 causing filament to fall off the side and failing yet again to print enough spool housings to surround it, I wrote my own in OpenSCAD. Now that it's in OpenSCAD I can experiment with different sizes and dimensions. I can also do cool things like putting an embossed logo on the outside to add some style. :)

Some changes and options I added:

Caps - you don't really need to completely enclose the spool, you just need to enclose enough to prevent filament from falling off the sides, you can use caps as spacers to save time and plastic.

Added a lip to the bottom to try and prevent filament from bending the housing and slipping under the spool.

You can change the thickness and the degrees that each slice of housing encloses to make it much easier to print cleanly.

Option to print a slice with a hole in it to tuck filament into when you change spools so it doesn't unravel.

You can change the tolerance to make it fit tighter or looser on the spool.

Option to print a slice with a nifty embossed MakerBot M in a circle logo (or a plain solid one). Use it as an example to make your own customized spool.