Use what you can. Kitchen counter for the Incline Pushups. A solid chair for the knee tucks, or sit on the top of the stairs. Don't get too wrapped up in what it is supposed to look like, but figure out what it is doing for you.
I did knee tucks on a chair, incline pushups on my desk, Australian pullups on my homemade suspension trainer. When I do leg lifts I hold onto a 25-lbs dumbbell to keep my torso in place while I lift my legs.
When I start getting into the uneven squats and pushups, I'll probably use my suspension trainer rather than a ball.