After much more cog turning in my little brain, I came up with a way to cut the longer side chamfers.
I would make use of the previous posts Table Saw Tenoning Jig: Part 2, but I would build on it further, to create a long L shape jig to be placed up against the existing fence, to keep the slab vertical and stop it tipping sideways.
After positioning the fence the correct distance from the blade, I cut into the L shaped jig part way as shown above (after testing the spacing and depth etc), then I shut the saw off and clamped it down.
This gave me the vertical support I needed to rest the walnut against vertically and it also prevented the tipping and all I needed to do was slide the walnut along the new vertical fence, while keeping it pushed against the fence at the bottom.
So I crossed my fingers, turned on the dust extractor, fired up the table saw and went for it.
I’m very pleased to say, it all worked out!… I got the 30 degree chamfer added to each of the long edges to match the shorter ones.
My Woodworking Story 