This ancient history thread explains some of the twists an turns in this story and mentions Casper:
Different Approach to DIY IMSB Retro / Parts and Pics
The problem with the freeze plug idea is the interference fit,the tool to install it, the lubricant(Bearing Seal?) -the issue is not axial of course.It is linear. If you force a very tight freeze plug into flimsy tube and it goes in like some RMS seals do,so you push harder ......
Try to adapt on of the IMSB insertion tools to do this gently & square ?
Recall that the IMS tubes are often oor so that may complicate the interference issue.
* Why do you think most IMSB kits use 1RS and no plug ?
Even the kits designed by JR and sold by LN. And those guys know all about the freeze plug concept. They deliberately do not use it with ball bearings but they do with the plain bearing. The cost issue is trivial.