The first thing that comes to mind is to get something more solid to brace your front foot against, maybe another rocket?
I'm a big fan of a plywood floor in the rower's bay,just makes river life so much more comfortable, and will make bracing in a little bit better.
What do you have in the drop bag under the seat? I run some ammo cans under the seat of my small cat to provide my back foot with a good solid thing to brace against.
Could fill in next to yourself for sure, Captains boxes would be sweet! could also add maybe some milk crates of beer. I think with a good solid brace for your front and back feet, along with a solid floor, you could be braced in nicely, try to think of having three points of contact when your rowing, your front foot, your back foot, and your oars. Being able to stand up and scout is really good in the Grand, I found myself doing that a lot when I was down there. Also good for when you need to stand up and push your body weight into a stroke for all your worth!
Have fun! Every day is just kinda mind blowing down there.
We can't always agree, but we can still be civil to each other.