What is the Best Way to Keep Digestive Biscuits Fresh
You're thinking along the right lines. Airtight is what you're aiming for. Old fashioned biscuit tins were probably less airtight than a lot of modern plastic boxes anyway, so just use the smallest tupperware box you can fit them in (all types in together is fine). I'd leave them in their wrappers, but it doesn't matter.

If you don't have any container at all, they keep for a few days with the end of the wrapper twisted round and clipped shut; that's enough of a seal.