This is an OpenSCAD parametric model for making THE timing pulley that you need. It's designed to generate a model printable without support material (observing the overhang rule of thumb).

You can configure it to fit the belt that you're using. The pulley can be fixed to a threaded shaft with a nut. On one side the nut (the setting for an M8 hex nut is configured for you already) will sink into the pulley. If you tighten a nut on the other side as well it will be very well fixed.

Another option is to use a set screw. You just specify the diameter of the screw and it will create a hub with a (capped) teardrop hole.

The STL file included works well with the Synchroflex(R) timing belt,1500Lx10Wmm from RS (order code 474-5892), spliced in half (to 5 mm width). This way you'll have a very good belt driven system for about 4 euros per meter. The belt comes as a closed loop, but you may need to glue it to get it to the right length.

Please share your settings in the discussion when you've tested it in combination with a commercial belt!

Below are the parameters in the beginning of the OpenSCAD file.
// //////////////////////////////
// USER PARAMETERS
// //////////////////////////////

// Pulley properties
shaftDiameter = 8; // the shaft at the center, will be subtracted from the pulley. Better be too small than too wide.
hubDiameter = 22; // if the hub or timing pulley is big enough to fit a nut, this will be embedded.
hubHeight = 8; // the hub is the thick cylinder connected to the pulley to allow a set screw to go through or as a collar for a nut.
flanges = 2; // the rims that keep the belt from going anywhere
hubSetScewDiameter = 0; // use either a set screw or nut on a shaft. Set to 0 to not use a set screw.
numTeeth = 20; // this value together with the pitch determines the pulley diameter
toothType = 3; // 1 = slightly rounded, 2 = oval sharp, 3 = square. For square, set the toothWith a little low.

// Belt properties:
pitch = 5; // distance between the teeth
beltWidth = 4; // the width/height of the belt. The (vertical) size of the pulley is adapted to this.
beltThickness = 0.65; // thickness of the part excluding the notch depth!
notchDepth = 1.8; // make it slightly bigger than actual, there's an outward curvature in the inner solid part of the pulley
toothWidth = 1.4; // Teeth of the PULLEY, that is.