Lots of thermal materials are available in sheet form, just cut to size and whatever holes you need. Some can even be laser cut (like Laird's kapton-based thermal films) if you have access to that sort of equipment somewhere, but generally an xacto knife will do the job.
If the transistors are insulated packages as in the photo, you don't necessarily need an isolator, silicone thermal paste (or equivalent) to ensure good thermal contact is all you need.