I gotta admit, I've never heard of an "umbrella scrim" until I just looked up what you were talking about. I just call them umbrellas, and I also don't use them.
However, I do have a bunch of rectangular and square scrims from different companies with different fabrics for diffusion, reflection, and subtraction.
The most practical ones are the Chimera metal frames that fold up. They're 5/8 inch thick, so the frame itself goes right into the grip head. They have fabrics, or you can tape on any gels, like Rosco diffusion. They range from 24 inches square, to 42, 48, 42x72, 42x82, and 6x6 foot.