How to install O/D plug

So what’s the preferred way to get the core plug in the counter shaft hole on the 833OD? I have the seal kit from Brewers and that plug is pretty thick and convex… it currently fits into the hole with no interference. I assume it’s meant to be “flattened” into place snug but it seems like you’d have to hit it pretty hard to make that happen… I’m hesitant to do that with the aluminum case. I was considering pre-flattening it and measuring with a micrometer until it just taps in snug…. Thoughts, ideas?