You only need about a quarter of it for your next batch. In fact it is better to only use a portion. If you have too much yeast, there will be little reproduction and few new yeast cells.
Suggestion: Get some Mason jars (or similar containers), sanitize them, swirl up the cake, and pour into the jars. Cap jars and store in fridge until you use it.
Cooler helps it store better.
As I said, you will only need about a quarter of the cake, so you don't have to store all of it. I usually get as much of the liquid out as I can, and end up with about 4 pints of slurry, so all I need to keep is a pint for the next brew (I usually keep 2 pints to cover 2 beers). In your case, since you left beer in there, your volume may be higher.
When it comes time to use it again, pour out the liquid and pitch the solids. Some people will disagree with me, but if the slurry is less than a month from harvesting, I straight pitch. If older, I make a starter.