I once had to sort out a Micra back end with a block of wood and a sledge hammer whilst standing in the boot.
Was crunched up badly by a van rear ending it.
Worked a treat and later wasn't picked up on an MOT. Was only just noticeable to the eye if you knew the backend of one well, very slight buldge where I went a little to far.
Takes time and patients to get it looking spot on, but can be done.