Clearly you want to wrap that with something that can withstand weather for a year and also expand if there should be much growth around the callus. Parafilm twice a year might do the trick.
I've seen what some folks call a step graft illustrated, in which both pieces have been cut in about 1/8 inch for little twigs to as much as half way through for larger, one or two inches from the lower end of scion and upper/outer end of stock, tied together with as much cambium contact as can be. Seems to be preferred with conifers. Haven't tried it, as have not grafted conifers and the other types of grafts I've done so far serve pretty well on apples: cleft, whip (and tongue, but not always including...) saddle and its inverse, bark or rind, side, and chip bud.
Funny how responses like this give one a reason to review what one has done. Three of the above I tried first time this past season. Even 50% success gives one the confidence to try other styles when conditions arise that appear to require a different approach.
Keep up the effort. It can change how you manage your fruits and, eventually, diet!