[Andy]

I made my own inserts. I forget what thread I used, but I tapped the bowl out to fit. I used a fine thread bolt. I drilled and tapped the bolt with the correct tread for the seat fitting. The fitting still sealed on the housing as the insert was below the boss. I did not finish tap the bowls. I left some untapped length so the insert could not screw thru.
I did quite a few this way. Never a problem.