This idea is simple and pure genius.
I would suggest this be allowed as an account only item, with up to 3 of these blank storage keys--20 slots each. One because your play on Evolution, one for a quest reward, and one because you donated.
I would also suggest a [House Version] with 50 slots which costs X amount of lockdown spaces, where X is---possibly-- 10 lockdown spaces per storage slot in use. (that's 500 storage space) This item should also be an account wide house add on. Just because you have 3 or 4 house plots doesn't mean you get 4 of these lovely ideas! Only 1 per account.
Also, allow the owner to reset the storage slots of this item by destroying it with an axe--like a trash barrel. Doing so deletes anything left within, but allows the owner to create a new house storage device (key) for free 1 time every 30 days. If for some reason a house owner needs/wants to reset this storage device a second time before 30 days is up, charge them gold or ED's. Be gentle with this cost of living though, if it is to rough at first it will fail.
EX: Free, 250k tokens, 500k gold, 1.5mil gold, 3mil gold, 5ED's 10 ED's
Well, there I went and blurted out my mind without checking the details again...
--Sturger--
P.S. I like this idea, but I do not think I will make it, since there are so many keys and quests related to those storage keys. However, I would love a personalize storage key. Great Idea!