They have a "custom size" option. I would just give them the dimensions of the Saker 762 and have them make one the right size. If you go with one that is too long and it hangs over the suppressor exit it will probably end up shredding the insulation a bit after you shoot it enough.
Suppressor Cover - Custom Size
I'm using their 9" suppressor cover on my Specwar 762 and it's just the right size. So for the Saker 762 you will prob need one that is 7.5" long.
It serves as a mirage shield. After about a mag if you are shooting any distance the mirage can severely affect your sight picture.
It also allows you to take the suppressor off before it's completely cooled down. It's too hot to touch after only 5-10 rounds depending on rate of fire, and it takes a very long time for something with that much mass to cool down. If you take it off it will cool down much faster by letting some air move though it and will allow you to get the rest of your gear put away without accidentally melting/burning something.