Much better to let a decent body shop look at it. I'm presuming it's spot welded like all modern quarter panels.
Leave the original on an get it sorted properly, preferably by a very competent man just like this fella>>>> (If you go through his vids you'll be amazed at some of his work on extremely busted cars.)
Filler is NOT for dent filling, it must be beaten out as best you can and use filler as a light skim coat to just finalise the contour/shape, no more than 3mm really.