Holding the position still makes it an isometric exercise, which is great for stability, but if you're wanting to increase weight there are a few options based on what you have available.
If you have access to a gym or have a few hundred dollars and a 3x5 sqft area at your disposal, this is called a reverse hyper extension, which there are machines setup to add weight specifically to this exercise. If you're doing this at home, you can try raising your flat bench a foot or two off the ground to give your legs more range of motion to go through, which will increase the difficulty, or do it off a box stand or other surface that allows you to dangle your feet. You can do this until your legs are dangling and not touching the floor. You can also try to incline the bench, which will make it harder. Keep your toes pointed down to help engage your hamstrings instead of your calves. I've also seen some people put a small resistance band around ankles or thighs to force a little bit of external rotation and make the glutes do a bit more work. Other than this, there is not a way to really add weight accurately, typically machines in the gym add weight by putting a bar against your calves with weight; anklets will not add weight like that. You might have a creative way to wrap a band around your calves to something below you underneath your bench, but I feel like the band would glide up the bench or up your legs, making it hard to keep good form.
To summarize, lift the bench up, incline the bench, and shoot for reps if you feel the work is too easy. Otherwise you can try alternative exercises.