This is my remix for this great educational toy. I remodeled it to have negative and draft characters, so no need for supports. Also no need for part cleaning, and the central tube is a standard 1" PVC tube.

UPDATE: I've included my version #2 for 32mm water PVC tube. This is the white and pink smaller assembly showed in the pics. Also prints faster (~3,5h).

UPDATE: Take care, version 1 is for 25mm tube, not 25.4mm. The modeled internal diameter of the wheels are 25.6mm . So if you use the 1" tube, please scale the part in X and Y to increase the gap, otherwise it won't turn.
Version 2 is for 32mm tube, wheel's hole is 32.6mm

UPDATE: Someone asked for a '=' wheel with '> <' signs so I did it for the 32mm version. STL available now.

I've printed in ABS 0,2mm layer, 10% infill, no suport, 50mm/s. Took ~4h to print.
Print 4 numbers, 1 operator, 1 egual, 2 end caps.
Cut a 1" water pipe with 139mm, assemble and superglue the caps.
All done!
Enjoy !
Paulo Blank
Mech Engineer
Brazil
UPDATE: For version #2 cut a 32mm water tube with 112mm length
UPDATE: Added the Solidworks 2015 files for the 32mm version (small)