i learned from industry that airtight is the key with as little of an air pocket as possible. less oxygen, means less chance of slime and bacteria growth. I have ro/di water stored in sealed sparklets 5 gal bottles and in emptied IO salt buckets for over a year. Had to use a few of the bottles recently with no prob. tds was still near zero. I used saran wrap to help form the seal on the sparkets bottles. the older style io salt bucket lids already have a pretty good seal. I chose the sparket and io salt buckets since they would require less cleaning effort. salt buckets are nice, since they are stackable. I would be wary of using some other containers such as soft drink syrup containers, not knowing how much cleaning would be enough. I would be concerned with leftover sugars and organics that could fuel slime and bacteria growth.
as a side note, my 30g rubbermaid container for ro/di topoff water requires draining and cleaning every month or so. even though it sees only ro/di water, slime builds up on the inner walls. the lid is loose fitting, not tight. after cleaning, i usualy add a bottle of drugstore H2O2 into the container's first fill to help breakdown any residual slime/organics left from the cleaning. Bleach could be better, but requires more effort.
(in the corner of the kitchen lol=/ )