it must be available somewhere but never found it, so when i was making them i used the side off-cut of the flag material, cut a strip any width you want fold and running stich 10 mm in from the open edge, create the pocket and edge to sew the flag to.
feed the flag edge in between the flap and sew with zigzag stitch feeding in the flag material 2 3 in a time gets you round the ark shape with no creases, reinforce the top to stop the poll pocking through. nice stitching with a contrasting colour looks good.
sew a loop on the bottom nice coloured string as for tension to finish.