Here,s how i do it on a jig I cut from MDF using a table router. See below - The jig is screwed to the neck blank. I dont like perspex as it is fragile and damn expensive round here. Obviously The router rides on top with a 1/4" bit through it to cut the slot,
For what it's worth that stewmac truss rod is not applicable to a front adjust neck as too much wood is lost at the adjustment point. This is a very vulnerable spot and where most head stock breaks happen. As a heel adjuster its fine but ensure you cut tele style channel in the back of the neck pocket for access with an allen key.
To cut that larger hole for the adjuster head a doweling jig is the way to go.
I bought a couple of them when they came out as they looked like a great idea for heel adjusters but felt the weld at the end opposite the adjuster looked a bit dodgy and I would have had to strip off the plastic to inspect it. I could have re wrapped them in shrink wrap I suppose but knocked them out for not looking quite right in a critical area. I have not bought them again.