I can’t find the specific episode (probably because they eventually re-run everything on YouTube, so it’s hard to guess the seasons), but America’s Test Kitchen recently did a thorough spot on sourdough starters. The closest I can find is recipes on their website, which probably served as the “source material” for the aired segment. While I don’t have much personal experience, the “master plan” and “myths” articles both look solid and don’t require a subscription.
That design looks more difficult to work with than the half-gallon and quart containers that we get in most of the United States (those are waxy cardboard rectangles with square bases and peaked tops), but a lot of people have used them for molds to make block-y things, whether literal blocks, candles, or something else. The waxy coating on the inside made everything easy to remove.
Those, though? As long as there’s no smell left, I might suggest just using them for storage. They look well-made for anything from oil to beans, since they’re not going to let air or light in.