I got a huge hank of oakum and some Life Data clay mud at the farrier supply store. They were out of Keratex hoof putty.
I used a mix of clay and oakum in the gaps. Then I put some clay in the frog grooves to see how durable it is. It doesn’t seem to dry out, but most of it stayed put over the course of a ride on sansy gravel trails. I did try cotton in the past but it didn’t stay put.
The oakum plus clay seems quite solid.