A plank on its side that targets the muscles down your flank. Start on your knees if the full version's a lot — that's the smart version, not the lesser one.
Every move in Core Buddy has a video like this one — and it builds the whole session for you, timer and all. Free to download.
The side plank with your knees down. Same shape through your middle, steadier base. Work with this version for as long as you like — when the full one feels within reach, give it a go.