Plus 1 on this. I just made my 1st conversion. And the Pentel comes in 3, 5, 7, and I think 9. Just search on Amazon for pentel pencils .3I have only done a couple. There are a few in kits in .5, but none in .3mm or .9mm.
When I needed to make those sizes, I had to go to the pentel conversion. Punch that into the forum search and you will find some good references and discussions.
Here is a couple
Your right, this is a critical area. Here's how a buddy of mine does it, and I followed his idea.Basically the pencil mechanism relies on the 0.25 mm angled "wings" at each hexagonal corner to seat. (4.45 - 3.95 mm) / 2 = 0.25 mm. The wings are not fully supported, either. The total cross sectional support area seems very small to me, and I wonder if even very dense hardwoods are strong enough to hold that lip through heavy use. That's why I thought of using brass washers as a seat for the mechanism. We'll see.