Many years ago I purchased one of those handles from Wunderlich. No matter how hard I tightened it to the frame, it still twisted around the tube every time you applied any pressure to it. Which is not what you want when trying to support the bike for moving it. The handle needs to be solid and secure.
I ended up getting rid of it.
Drilling the hole is easy. Undo the 6 screws securing the filler cap and remove it. The plate holding the filler neck can then be teased out of the tank by lifting it from the front end and angling it up so that the plastic overflow valve can be pulled through, there should be enough play in the fuel hoses connected to the filler neck to allow you to do this. Once the filler neck is out, you can mask off the hole to the tank to prevent dwarf from getting in. Then drill the filler neck. Refit is the reverse of removal. You may find the O ring has swollen. If it has just put it somewhere to dry out so that it shrinks back.
Even though I have the hole drilled in the filler neck, I seldom brim fill the tank unless I'm going to burn the fuel off. Otherwise the fuel just ends up on the floor coming out of the tank vent. I think the long neck is a design feature to allow an air pocket in the top of the tank to allow for fuel expansion, without that air gap, the tank will become pressurised.
Ian