Note that there are many automotive dentless repair vendors who slide their tools into all sorts of difficult to access cavities in cars so that they can massage dents out that look like this. We have had some dents that looked worse than that massaged out in spots that had very tight access. They usually have a very large and varied assortment of massaging tools and some of them have mad skills. You might want to see if one of those guys has some tools that will reach that far.