I too work at a shop that does repairs. we use the plug/patches pictured above. I've fixed dozens of tires with the external plugs that were leaking. seen a few that were fine but all by the same farmer. Never had a customer come back that I know of in the year and a half I've been there.
been to a shop that did external repairs like you guys but we're not gonna go there. that was the least of the issues there.
1991 Toyota Pickup 22R-E 2.4 I4/5 speed
1990 Toyota Cressida 7M-GE 3.0 I6/5-speed manual
mechanic, carpenter, stagehand, rigger, and know-it-all smartass
"You don't get to judge me for how I fix what you break"