Hello,
Are you sure the sloshing is from underneath the seat? Sometimes, when my pagoda gets caught in a down-pour, water flows into the soft-top compartment, and then through the compartment drain holes and into the space underneath the rear shelf (behind the seats). You can check by removing the rear shelf pieces (or the kinder-seat if you have that option). I haven't been able to completely seal the soft-top compartment, so whenever I get caught in the rain, I always open up my kinder-seat compartment once I get home, and drain out the water.
WAQAS in Austin, Texas