Farmer's and 1 arm farmer's are cool, but sidebends will do if space is tight. Focus on neutral spine and tension in your oblique. Don' t worry about bending too far, it only takes a short ROM.
I like pull down abs with a band. Also hanging leg raises. Or leg raises off a bench to start out. And the side-to -side "landmines." Lifts that you can load, so you only need 8-12 reps. Stuff where you stand on your feet and don't have to lay on the ground.
For the posterior I like the back raise or hyperextension. Hug a ball or plate to your chest. Tuck your chin. Really push your hips in, and use your glutes and hamstrings, not your lower back to do the move. When you get stronger, put the plate behind your head.
Leg curls, kettlebell swings, glute/hams, pull thru's, reverse hypers can all work too.
You don't have to spend much time on this stuff. 3-5 sets, 10-15 reps. Before your workout, or between sets of assistance work later in your session.