Maybe to late for my two cents. But you could fill a double trash bag with sand (not so heavy that you can not carry it or it breaks); make sure you have most of the air out when you tie it and make sure the sand is not all balled up. You want the sand loose enough to spread out over the main drain (of course the sand is in the bag). Then cut the drain pipe. If there is to much water comming out to glue a fitting then use an air inflated test plug with a chain attached to plug the pipe. Then glue on a good valve (Jandy, Compool or equal). When you do this make sure the chain to the plug comes out the other end of the valve. When the glue has cured then hold on to the chain and deflate the plug with a long screw driver. Not a big fan of putting valves in the ground but if you do not want to drain the pool down this works.