I did a bit of development work for a company that made replacement rear axle shafts for quad bikes, just like you want to do . The splines we had on the shafts were a 90 degree angle and were rolled onto the shaft originally. we were able to cut the splines by using a dividing head and a simple end mill along with a bit of relatively simple maths.

I recently had to make a clutch shaft for a modern clutch with square splines. I wasn't till I started to look at the form that the teeth are square but the cutter needed had angled sides and a radius on the bottom of the cutter. I managed to gang mill the teeth and used an end mill to under cut the radius.

In theory a straight sided spline locates, and drives on, the flanks of the splines. Neither the OD or ID play a part. However, unlike involute splines straight sided splines are not self-centreing. So depending upon tolerances the OD or ID may in practice provide a limit to radial movement.

Rather than try and measure the angles it'll be much simpler to do the maths, or draw it out in CAD.

Commercial cutters are available for straight sided splines, but tend to be for larger splines than MEs would require. When I needed to cut the splines on my traction engine crankshafts I made my own spline cutter:

The splines were cut on the horizontal mill using a dividing head for indexing:

Note that the mechanism is not rusty, it's a trick of the light fooling the camera!

Andrew