1) remove the aperture stop so the laser inserts farther (the aperture stop is not necessary in the Tublug).
On the other hand, the Tublug works just fine with the aperture stop in place. It just needs to illuminate the center spot and a little more. The image is dimmer but that's about all. I have one of the older 2 inch Glatter collimators and the constant threading and unthreading of the aperture and stop and self Barlow wore out the threads so I need to be careful...
I can attest to the fact that it works just fine with the 1mm aperture stop in place, even with scopes with focal lengths of 8ft and 10 feet. If one is trying to collimate in broad daylight, then the brighter image would be handy. My back up laser is an 20 plus year old Howie Glatter 1.25 inch. Back then, the aperture stop was not removable. It works with the Tublug.
As far as the 1.25 inch versus 2 inch Tublug. I would go with the 2 inch, it's one less interface. With the 1.25 inch Tublug, you have the adapter, then the Tublug, then the Collimator. With the 2 inch Tublug, it fits directly in the focuser, it's a nice "slip fit".
I remove the bottom nylon set screw because the collimator in the Tublug is like a piston in cylinder, it builds up pressure if that bottom screw is in place.