that idea actually was from tom barr, used for osmocote ferts. Another idea would be to sandwich the mts mud between 2 piece of plywood, shove in under gravel, lift and slide the top plywood to allow gravel displacement on top of the mts and second plywood. Then gently press on the gravel and slide the second plywood out slowly. This will leave the mts at the bottom of gravel. I don't know how well that is going to work out through. Instead of plywood, maybe you can come out with something rigid with less roughness.