Go to an outdoor/camping store and look at the dry sacks. Designed exactly for this.

https://www.osprey.com/gb/catalogsea.../?q=dry%20sack

Get one with a window if you can, hopefully it will remind you to empty it as soon as you open your retrieved suitcase.

Since you are thinking about building up a collection of packing cubes, this is just a variant, potentially part of your packing cube collection.

If you want specific advice on packing cubes (I find these indispensable by the way - yes, they save space) there is a forum for related questions:

https://www.flyertalk.com/forum/travel-products-639/