I did a throwaway graft so I could take some pictures.
Its a variety I wont keep on this tree even if it takes and I did not wrap the scion. With the disclaimers out of the way, this is what I was trying to explain.
First the root stock is a mature tree that happens to have some CAC branches with thin but hardened shoot. In this case it is hardened and stiff enough that I did not need a splint. Your seedling may not be in that situation.
I took a branch, not prepped from a tree I was going to cut back (2nd year of a 3 year topwork exercise removing a third of the canopy each year)
The first two pictures below is how I made the cuts on the scion.
Notice one side flat, almost like a veneer graft and the other deep. The flat side is about the width of the shoot on the root stock.
Next is the cut I made on the rootstock shoot. Almost to the halfway point to get as much width as possible. Note if this fails, I have lots of branches. Were this my only site, I would cut only to about a third of the diameter.
The next two show the scion next to the shoot. Notice the cut on the flat side is about the same as the width of the shoot and the cut on the slanted side is much wider bt not as long.
A quick test fitting and a little wrap to hold it in place long enough to take a shot. Notice it stays in by itself, no support. The scion is clearly wider as you can see.
Note the 3rd and 4th images, they show the slanted side and while I pushed the flap to one side as much as I could I am not counting on cambium contact along the edge but if you notice, I cut the flap long enough that it crosses the arc at the top so there is a chance, however remote that I may get contact on this side of the scion. The other side, the flat side though is what I am banking on. It is a close match and the chances of cambium contact is higher.
Wrapped up and a clothespin to secure in place.
Were this a graft I really wanted, the scion will be prepped and will be wrapped to prevent moisture loss. In this case I will just place a bag over it.
You take a shot and share your pictures. Worst case you will have two high likelihood on your pencil thich shoots and two fair shots on the thinner shoots.
One more thing, I did not remove the tip of the shoot. In a graft I want I will but will leave some leafs on, jut remove the terminal bud and a couple of leaves.
One last thing, I saw Adam (Flying Fox Fruit) do this thing where he makes a little H with buddy tape and uses it to seal the gap between the scion and rootstock to minimize moisture going into the graft (see 6:40 on
https://www.youtube.com/watch?v=sIx-kvKykec&t=756s). I often will do that with these side clefts. I don't know if it helps or not but I liked the idea and adopted it.