I've had for many years a T handled screwdriver to fit the engine case screws on these engines and as you say it makes a big difference to working with them and has never damaged the heads so almost everything is easier to work on with it.
If you wanted to hold something like the throttle slide as in your original post you can make a quick jig by boring a hole the right size in a piece of 4x2 softwood then cutting a slot through into one side of the hole, when you then put the slide into the bore you can either pinch it across the slot with a woodscrew or clamp it in a vice. It will hold it really tight and not damage it, that goes for anything cylindrical really. If you think about the top clamp on the front fork leg then that's the sort of thing you'd be making.
At least you've got it sorted anyway.
