Hey Jim, I echo Boyd’s suggestions using the Amazing Clear Cast. It is a slow cure, overnight. I’m in the middle of a bowl project that involves a large gap in the crotch area and if left as is, would not be confident turning it.
Some key things to be aware of once you accept that this is not solid check free wood, are following.
1- The wood should be dry/stable so it doesn’t move and potentially lose bond to any filler.
2- The success of the fill will be much better if you remove all punky/loose/rotted wood. You want the epoxy to bond to solidsolid wood. I spent a lot of time with dental picks and pliers to clean out the 10”x1” crevasse.
3- Use some sanding sealer or dewaxed shellac to seal pores of wood where the epoxy will come into contact, this will help prevent bubbles from the pores.
4- Amazing clear cast works well with Alumilite dyes and pearlescent pigments, I think mixing in the part B which is heavier is best so you can give bubbles a chance to rise before mixing in part A.
5- Lastly, try to pour in a way that doesn’t trap air that will show up as a pocket.

Good luck and be safe!