If it were me... and I also have an overflow. I would first undrestand that the weldon bonds the acrylic, much the way welding bonds steel. I have though long and hard about how to remove mine. I used to think maybe a dremel, but this would hack the job. after considerable thought, I would use one of those very flexible wood saws for flush cutting wood dowels. I would use a shim strip about the thickness of a credit card on both sides of the pc to be cut out. I would then slowly work bottom toward top, keeping the blade flat against the shims, once removed, I would dremel the bottom, as it is not as important.
I would use 2 different scrapers to remove the ridge left, one that is nearly flat, one that is flat, like a razor blade. be very carefull not to gouge the pc of acrylis you are trying to save. When you get really close to completely having scraped off the high points, swithc to the scratch removal kit and the sand slightly to remove the small particles. work your way up to a 3200 grit or so, then switch to a buffing wheel and the novus 2, this should restore the finish.
I have not attempted to do this in my tank, as it would be extreamly time consuming, and the margin for poor results or distortion where the pc being removed from is very high. I would recommend trying it on a pc not in a tank first. Acrylic si so dang soft and so eash to scratch and gouge.
Why do you want to save the box, life would be so much easier to build a new one if you really needed it.
Jason