Site Lite SL-100 Laser Boresighter, as new in box. The best, easiest and safest method I have used. I have two, only need one. Strong magnetic arbor, centred by o-rings avoid contact with bore, and give reliable results.
Link:
https://sitelite-lasers.com/product_SL100.html
$75+$20 shipping if required Located in Medicine Hat.