Since webform, for showing results implements a different path than the standard node/%, you need a way to tell Panels to take over this path.
The module Page manager existing pages allows you to do this.
Install and enable the module. Go to
admin/structure/pages/existing-pages/add, find the path
/node/%webform_menu/done and add it to the list.
Now go to
admin/structure/pages and you should see a new "task" for results, where you can customize the result page.
As pointed out by winchendonsprings, the results are available under
Add Content >> Existing pages >> Existing page >> node overview.
In order to do this on a webform by webform basis, one will need to add a manual node context, where you select the webform you want by nid, and then add a selection rule that compares the current context against the manually loaded one.
Normally manually loaded contexts should be avoided as "hardcoding" node id's is rarely a good idea, but this seems like a good exception from the rule, although it will still not be properly exportable.