Mead, you done a pretty fair job with it so I wouldn’t worry to much, and like Bravo said, once you get in with the sanding it evens itself out nicely.
I had to cut the bottom part from mine to ensure a nice snug fit as you can hopefully see below.
The cut out and after a prelim of sanding back.
I don’t think you can ever make surgery like this pretty, and anyway all the gouged areas and cut marks will be hidden … I found it easy to in sections and from the bottom up with my Tamiya saw as it gave me a bit more area for blade grip. I then trimmed with a knife blade …
This is only dry fit and held in with tape. The gaps you can see here all practically go when you push it in and it is a pretty good fit considering it’s not the AFV club wagon it was meant for. Hopefully filling will be a minimum effort as well…
Just have to fully sand back the moulded on retaining strip and should be ok I think.