Bloc Loc has a good one. It has a grove that catches on the seams to hold it in place. I have the full set up to 8". Thankfully I bought them quite a while ago. You can just buy the size you need or get a set. Different sites sell them, so check prices/sales.
Bloc Loc Flying Geese Rulers I use Bloc Loc Rulers for a lot of my trimming, they have all different types from half square triangles to log cabin rulers, etc.