I am using Balsamiq Mockups for years now. Since I use Webflow, I tend to skip the real mockup step, going from paper straight to the designer. But for bigger projects, and for projects I only use Webflow to make prototypes, I still rely on mockup tools. And when I can chose the tool, I go for Balsamiq.
So I made a grid system taht mimicks exactly the one you find in Webflow. Same amount of columns for each breakpoint, and same scale from a grid to the other.
Brendan from Balsamiq has accepted the grid systems on the resource site Mockups to go, so you can download them as BMML mockups (File > Import > BMML as mockup)
There's a version with devices and one without.
Here you go, they're free to grab of course. Don't hesitate to reach me if you need any info about Balsamiq or anything else.
If you're new to mockups or balsamiq, here is a quick preview of what it does:
More mockups at the end of this page, and associated designs: