2 1/8th inches, it is flexible enough to stretch some or use a zip tie to make it tighter.
Mr Ray, Does your vortex eye guard screw off? If it has threads you can use step up or step down rings.
The eye piece has a protective rubber shroud (eye cup) glued onto a grooved metal sleeve. The grooves ride on two studs allowing for adjustment of the eye cup relief. I took it apart looking for threads, but found none.
The outside is 47mm which is just shy of 2" so that lens piece would work once the cover is off. I'll add one to my T2 list.
Thank you