I need the lowest density possible in a paperboard of approximately 60pt thick. It does not need to be "printable". My application would be to mount/glue printed sheets to each side: this 60pt is just sandwiched between other layers to make the end result thicker. It needs to be ultra-low density because it will be diecut into intricate shapes (think similar to jigsaw puzzle pieces). So, could be chipboard, blotter board or similar, but must be a solid board as opposed to corrugated.

Any ideas on what to specify or where to find?

At 60PT you're looking at something in our Rising family of conservation boards. This thickness equates to a 4-PLY. However, as these boards are produced by laminating 1-PLY sheets together the density will be quite high.

https://legionpaper.com/rising-conservation-board/

It might be better to simply go with a gator board or foam core.