Which is more efficient: Trying to unbox an unusual [from series 18+], or trading keys for one?
if you use the keys you can unbox 1,000 crates and never find one, or get one on your first try, it's all luck
the safest way is to just accumulate keys and use them to buy one, it's not nearly as exciting but it's also not as potentially disappointing
but you'll need at least 20 before you can get yourself a decent one