Had that problem trying to use an old 5000Di a long time ago.
Take the coil off the shaft lay it on a table or bench covered with wax paper. make a cardboard ring that fits around the coil with 1 1/2" space, and at least 1" higher than the coil. Cover the coil with saran wrap. Pour rubber sealant like SPL over the coil. Let it cure, when cured take the ring off, lift the rubber off the coil, cut out the hole for the shaft and cable. You can trim the rubber down until it just holds the coil at neutral bouancy. the coil weight will stay on the poil, and can be slipped off when working the dry sand. If the coil doesn't stay on, get one of those tiny suction cups from the dollar store and silicone it into the top of the weight between the cover & weight. Mine worked well, gave it away to a friend when I moved to Colorado. If you've got questions I can make a drawing for you.