My understanding is that the insert should have a very slight interference fit with the counterbore. Not more than .001 or .002 I think. Press the new seat in with the outside chamfer down so it doesn't peel any chips off the wall that would intefere with seating and subsequent heat flow from the valve to seat to head (or block). Chilling the insert before insertion may help too. I'd peen or roll the top edge of parent material onto the new seat just for insurance. This might require the counterbore to be .015 deeper than the insert thickness. I'm told this is even more important using high chrome or stellite seats since they don't expand at the same rate. IIRC I've also seen loctite used on valve seats but I've got some reservations about that myself on the theory that it might slow down heat flow and lead to burned valves. Maybe Onan doesn't run that hard but I'd try to give any job the best advantages.
"In theory there’s no difference between theory and practice. In practice there’s a lot of difference.” Yogi Berra