Thank you for the recommendation! After deliberating back and forth over whether to go with this or a DIY solution, I ended up buying this one.
I saw a tildes comment talking about this adapter. I'm thinking that, once we finish the first couple of refills, we can decide whether we want to get a bigger donor cylinder to refill them through this.
And then, if this ever does break, we can switch to a full DIY solution at that point, since we'll then have more hands-on experience with the larger cylinders. So, we can gradually ease into a more DIY approach once we've shown we'll stick with it, and have a bit more confidence.