Need suggestion for covering hole

So if I am picturing this correctly, your talking about a base cabinet that has a sink just like the one posted above but the hole is in the side of the cabinet as you down the stairs. Depending on how the plumbing is laid out and how much you need the storage below that left hand side of sink,you might look at putting a cubby shelf built into the cabinet similar to the one the CD's are in in the pciture. It would be flush mounted so no issues with the step and give you a place to stash stuff that you use outside but dont want to have to come all the way in for. :Like a small mag light flashlight, a lighter for the grill/stove, bug spray, etc. or maybe you decide to make it bigger to stash something bigger. You could also make or buy a matching cabinet door to your cabinets from the dealer/manufacture to have a covered storage spot there.

Good luck in deciding your best options and needs.