Just a thought, but as an algebraic type this is an exponential, so the following could work:
[Int^5]
[[Int^5]^3]
Or:
[Int**5]
[[Int**5]**3]
However, this reverses the order and I can imagine that is not preferred. We do have an existing notation for exponentials that does use the proposed order, which is the arrow:
[5 -> Int]
[3 -> [5 -> Int]]
This may be too theoretical, though, and perhaps confusing.