Any air tight bottle or container will do. You can reduce with Illustration base for storage. Rule of thumb though is don't pre-reduce and mix more than you will use is say a month. I use the little one ounce mixing cups with snap on lids if I need to pre reduce for an ongoing project. But I try not to pre reduce more than I will need.
With regular createx , if you are just premixing custom colors , There is no need for the illustration base. and never reduce until ready to use when it is in the cup. Due to storage temps and temp changes in the storage it can cause the paint to go bad.
But as choppa stated air tight bottles work great.