Sevcon Gen 4 - Heat Sink Insulator Material?

I repaired a Sevcon and need to re-assemble.
It appears that a green material is used on the heat sink... obviously a thermal conductor and electrical insulator.
It is soft enough that it has "smeared" a bit onto the back of the TO-220's - I thought that was very weird.

I could re-assemble with the existing insulators... and it would work... but its for a high reliability application and I dont want to risk it.
I could of course use Kapton or some other insulator... but I am hesitant to change the thermal characteristics of the controller.

In my experience I have seen:

* Gray thick woven material - in the old ebike controllers
* Kapton tape in various thicknesses
* Dry Kapton with no adhesive

This assembly is particularly gnarly because it is spring loaded with LOTS of fets... very easy to scrape a corner going in and get a floating pin-hole...
A pinhole could pass a continuity test and fail hi-pot later.
I am hesitant to hi-pot a customer controller... but we do have a 350V supply so I suppose I could do it at 150 or so.

As for a jig to load the fets... I dont have the time or resources to build a proper jig so I will probably bring them in at an angle and drop the three sets down step by step.
There are 6 spring rows to unload... and "pushing through" once the lip is clear looks dodgy at best.

-methods

It's a thin calandared Nomex paper with a Silicone based thermally conductive particle loaded soft layer facing the FET drain tab side. I don't have any spares, but I can bring you a few scrap broken sevcons taking up good garage space and some thicker and not as good thermal pad material you can try with.

My only time getting one back together successfully required 12 long flat blade screwdrivers loaded into all the spring clip tabs to keep them sprung back, then after the FET assembly is in place you pull the screwdrivers out from the sides. It's very awkward and easy to botch.

I believe it.

Thanks for the details.

The PCB is dead nuts simple and easy to repair.
The assembly is FUBAR.

Pavl could no doubt knock together a jig while shooting the shit at his shop.
If I find some money for him I may get him to build one then post pics of it for others to duplicate.

The Sevcon is no longer a mystery.
I have not sat at a dyno tuning one for days on end... but I have core competence with it and could do so if paid.

My boat will definitely run on one...
BTW: Size 6 units for $300 in reasonable volume with Sin Cos

-methods