I strongly suggest to not use mica for mains-case insulation as it sometimes cracks. Thick-ish silicone pad is much safer solution. Also don't use screw+plastic washer. It's a very weak point. Use square silicone pad without any holes + clip. Or even better, use silicone cap over whole transistor instead of pad.
The only time I have seen mica fail is when it was damaged by being folded. It is pretty delicate but there are other insulating pad materials if something more rugged and easy to use is required. I have seen alumina crack even in normal service.
A clip to apply pressure at the center of the package is one of the best mounting attachments because it is least likely to bend the die frame but it can be difficult to get enough force.
The screw attachment requires insulation and tends not to have enough compliance but works very well if a Belleville washer is used. Split washers do not apply enough force. Because of poor availability of Belleville washers, I tend to use several wave washers stacked together to get enough force.
Motorola published an excellent application note discussing power semiconductor mounting considerations:
https://www.nxp.com/files-static/rf_if/doc/app_note/AN1040.pdf