This is the jig I made some years ago. It is a bit dusty as I have not used it for ages.
It is simply two bits of MDF, nailed and glued together. The guide is a bit of softwood, sized for the grove in my bandsaw.
The angle is not critical. What is critical is consistency which is what my jig gave me.
What can make the cross look out of shape are that the blank is not square and / or the hole is drilled off-centre.
The way I use the jig is to cut through the blank to a depth of about 90%, glue a bit of contrasting veneer into the slot, let the glue set, clean up the faces and repeat the process on the other faces of the blank.