Get some neoprene wetsuit cement and some silicone grease from a dive shop. For the procedure below use a q-tip as an applicator, but use a cheap off-brand, since you don't want a lot of cotton getting in your way.

1) clean the dust and crud from the boot and find the crack. Use a solvent like mineral spirits
2) apply some wetsuit cement to the edges of the crack, then move the actuator arm to the point in it's travel that keeps the crack closed
3) apply a thin layer of wetsuit cement to the entire outside surface of the boot, making a thin 'skin' over the entire outside. Let cure for 4 hours
4) repeat 3) to make a second skin
5) repeat 3) to make a third skin and let cure for 24 hours
6) The last skin will still be tacky and that's no good because the boot would stick to itself in the lock position, so take a small dab of silicone grease and apply it all over the outside of the boot. this will neutralize the tacky surface

done