A couple of ideas:
1- For ensuring that the Elevate 3's stay on the C-stands, use the two screw positions on the rear of each speaker. Make a short pair (1/2 inch wide, 2-inches long ?) of steel or aluminum 'straps' that have a hole near each end. By using a slightly longer screw replacing the original, you can go through the strap into the speaker. The other end of the strap accepts a screw that goes into the rear edge of the upper part of the C-stand.
2- If there is any overhang of the rear edge of the desk on which you're placing the C-stand, then a vertical slot in the center rear of the C-stand's spine (between the two lower triangular brackets) could be made to accommodate a 3-inch C-clamp as is commonly found in home improvement stores. The slot would have to be about 3/4-inch wide for the clamp's hooked end, and (obviously) the threaded circular post touches the underside of the desk's overhang. If the desk doesn't have an overhang, and it's not a family heirloom, you might be able to add a small block of wood to the rear edge of the desk to create an overhang.
HTH