Having read the technique story about using a 'Candy Tube with water' reminded my of the times when we practiced with a small sausage balloon. You place it over a slim dildo/vibrator to insert it in either hole or even both (great feeling) and then having removed the insertion item, put an average glass of water in the balloon with the aid of a plastic tube and then blow up the balloon as hard as you can via the tube, tie off the end of the balloon and insert.
It does not matter WHAT you do, the water will roll around and you will experience .... WoW .. the added fun later is when he tries to pull the balloon out. Actually, we may try it again now. Have fun.