I have this one saved in my amazon lists

Pelican iM2050 Case, Watertight, Padlockable Case, No Foam or Divider Interior, Black https://www.amazon.com/dp/B00OUF3WT2/ref=cm_sw_r_cp_api_i_NqAPDbP8WNXHA

It’s the one I’ve seen used the most. You need to drill holes in the bottom and I recommend lock nuts so it can’t be easily removed. It’s a lower profile but it looked pretty good on the S1 back rack.