Regarding a flange on the wheel pulley, YES generally the more flanges the better.
But technically speaking it is not needed, with such short center distances the suggested minimum is two flanges total, they can be located on just one pulley or you can have a flange on each pulley but diagonally opposing each other.
If you do have just two flanges and on just one pulley you need to make sure the pulley tooth width & belt width tolerance is fairly tight because the belts will always want to travel one direction due to the twisted strands of fiberglass inside the belt. Timing belts have strings of twisted fiber glass in them to stop them stretching and give them strength.
If anyone would like to read the 90+ page technical journal on belts & pulleys check it out: http://file.lasersaur.com/docs-thirdparty/The_World_of_Timing_Belts.pdf
Here is a quote from page 39
Because of a slight side thrust of synchronous belts in motion, at least one pulley in the
drive must be flanged. When the center distance between the shafts is 8 or more times
the diameter of the smaller pulley, or when the drive is operating on vertical shafts, both
pulleys should be flanged.
FOR EXAMPLE: The enertion motor mount center distance is approx 70mm, the smallest pulley (13T) diameter is about 19.5mm diameter. The center distance is less than 8 times (it is just 3.6 times) the diameter of the smallest pulley. So technically two flanges is sufficient.
I doubt anyone has a motor mount with 8 times center distance on an esk8
The other reason I chose not to flange the wheel pulley is because of manufacturing restraints. The flange needs to be pressed onto the pulley. This requires special tools… increases cost, etc.
Anyway, I personally don’t think you need a wheel pulley flange, you just need to make sure you have a motor pulley width that is the correct size for the belt.
Or as @Gatwod has ingeniously shown just use rubber o-rings!