I spoke with a BG tech at a local shop and he told me to spray 1/2 the can into the evap box, just open the door and put the tube in there. The stuff foams like heck and fills up the box. When it got close to the opening, I removed it and put the cover back into place. Ran it for 10 minutes without a/c on.
Repeated with the second half of the can and ran for 10 minutes with a/c on full blast. Nothing split and worked like a champ without drilling a hole. He also told me to finish by spraying the evap with water and running a/c again to complete flush. Hope that helps. I am not very skilled and it was very easy. I have to think that the drilled hole is for boxes that do not have an opening for filters.
Thanks for posting this. My LC is one of the ones that does not have an opening for filters. Still need to figure out where I can drill a hole safely.