As often, I'll be the contrarian here.
No stirrup work teaches you to ride from the knee up. Your weight cannot slide into and be supported by the stirrup, so all weight must be carried from the knee up. It is good for learning how to ride without stirrups.
If you want to learn to keep your stirrups, use your stirrups. Spend time riding while standing in the stirrups. Walking, trotting - two point reduces the peak pressures on your horse's back by 20% compared to both sitting and posting - and cantering. In addition to being gentler on your horse's back, you will learn to let your weight slide past your knees and into the stirrups. You will also learn to keep your stirrups under your center of gravity. You will very quickly realize it is much easier to stand in the stirrups if your center of gravity matches your horses, and will thus learn how your horse adjusts his center of gravity while turning, accelerating, slowing, etc.
Foot position: Get a ladder. Climb up and down it a few times. Notice where you put your foot on the rungs. Use a ladder with narrow rungs to help. The place you put your foot to climb a ladder is the spot on your foot that will give you your most secure grip on the stirrup. Put that part of your foot on the front 50% of the stirrup.
You are in Australia. FWIW, I developed a love for Australian 4-bar stirrups:
Prefer riding western now and like using stirrups with a 3" deep bed.
Full disclosure: I don't teach anyone. I started riding at 50. A nagging back injury means I'll probably never use my back well. And I mostly ride on trails and NEVER ride for a judge. Other than my horse. But I can't remember the last time I lost my stirrups and I'm suggesting what has worked for me.