Keep a maximum of 5-10 Slimes in a Corral or if more are desired to be added, invest in a High Walls upgrade.
There are a variety of upgrades available to assist with slime upkeep: High Walls, Music Box, Air Net, Solar Shield, Plort Collector, and Auto-Feeder. If a player were to fully upgrade a brand-new Corral, it would cost a total of 2550 Newbucks, not counting the 250 Newbucks to build a new Corral.
Increases the height of the Corral walls, so slimes have less of a chance from escaping via jumping.
Soothes Slimes with music, making them less agitated.
A force field at the ceiling of a Corral, prevents Slimes from escaping completely. Will drain very quickly without the High Walls upgrade.