For sitting the trot, or sitting the canter on a bouncy horse, here is what works best for me (and FWIW, I'm a nobody in riding):
Get your heels about 6 inches in from of your belt buckle. Adjust the stirrups so your knee is at the widest point of your horse's barrel. Go a bit toes out if needed, so the knee flows around your horse.
This does a couple of things. It makes it very hard to grip with the knee, and gripping with the knee creates a pivot point that allows you to bounce and come out of the stirrups.
It makes it easy to have heels down. While you are reading this, with your heel in front of the chair, lift your toes. They will lift plenty. Keeping your toes lifted, bring your heel under your hip. If you are like most, your foot will now be flat, or even a little toe down.
It also allows you to use the natural bend of your hips to flatten out and absorb the bounce. Look at the two pictures below:
The main difference between the cowboy at the top and the champion dressage rider on the bottom is that the latter has his leg vertical from the knee down. When the bounce gets big, even dressage riders don't keep a vertical line from shoulder to hip to heel.
This remains one of my favorite videos on riding. It is from a western perspective, but the basics work well in most any saddle:
Good luck! The more you get used to the 'bounce', the easier it is to adjust where you put your leg.