How to pull dents with no access from behind?

[72step]

I have 3 dents on my truck with no rear access for hammer and dolly work. One on the roof top, one on the rear of the cab behind the door as it wraps around and on the very front corner of the hood. I really wish I had one of those tack-gun dent pulling systems but I don't feel it's worth spending the money on one for these three dents. What other options do I have to pull these out?--I do have a mig welder. I was thinking of tacking a piece of metal I can grab on to and somehow making my own dent pulling rig. The dents are not major by any means. Any ideas?? Thanks...