Leah Day sells a quilting back support that puts a 2 pound weight in your lower back area and the pull from the attached shoulder straps remind you to sit up straighter and not curl your back and shoulders forward. (You can find it for sale
here, along with a short video that shows it in use and how it works.) If you have some small hand weights (or something similar) on hand, you could probably rig up something similar to see if the general idea is likely to work for you. I haven't tried this product, but I think it looks intriguing.