I believe casters lack popularity because, in general, they need to be ordered in advance and costing more than maggots the fear of wasting them if not used is enough to put an angler off.
A simple procedure, however, will enable you to store them in perfect condition for at least a week. Immediately I remove them from the plastic bag they were sold to me in and pour them into a bucket of clean water. A few usually float to the surface which are removed and thrown away. The rest are then drained on a pinkie riddle before packing into a small container until all that is left is a gap at the top of a couple of millimetres. Next, I cut out approximately ten pages of newspaper in the exact dimension of the lid and this is then dampened before placing on top of the casters.
Finally, over the top of this, I pull a plastic bin liner taut before snapping the lid shut ensuring that I create an air-tight seal. I then place the box on a concrete floor with a weight on top to ensure no air enters the tub. Left like this, the casters will still be in tip-top condition ready for my next session. Just remember to lift the lid re-seal every 24-hours.