I use stackable Twist N’ Loc round 2-cup containers from Ziploc, but the idiots at Ziploc decided to discontinue them. Thankfully I have a LOT of them. (I’m making corn stock right now from 1 dozen cobs from last year.)
Alternative BPA-free 2-cup size that looks exactly like the Ziplocs here:
We haven’t tried it ourselves but heard for large amounts of stock reduction that the oven is very good as you can run it overnight, makes more flavorful reduction and cleaner tasting
For stock I use the 2 cup containers from anchor hocking or pyrex. They’re just the right size for freezer storage and take about 3 minutes in the microwave to thaw. There are replacement lids available. Stock cools quickly when decanted into these so you’re not left with a giant pot that need hours. Even though they’re round, they stack well in the freezer. A tape label with the the contents and date slapped on the lid makes things easy to find.
A friend from a local cooking group I run came through and supplied me with a few cottage cheese / yogurt containers. The stock had reduced so much that I only needed one 32 oz. and one 24 oz. container.
I already had two 2-cuppers and four 1-cuppers in my downstairs freezer before I made the four 2-cup containers a few days ago. I’ll freeze the rest of the cobs in ziploc bags to make more later!