Open up the seam that runs down the length of the sleeve about an inch down from the top. Try it on again. Bend your arm! Don't forget moving is a thing that will happen! If this is enough I'd go with the tiny-button-and-elastic-loop option presented further up.
If it isn't enough keep opening that seam until you get to the place you want it to sit. Once you know how much it needs to open you need to measure the distance of your gap. if you don't have a buddy handy, tape the sleeve to your arm on both sides so it isn't flapping and measure the distance. Also measure the distance from the top edge to the bottom of your gap. Now use a piece of paper and draw a triangle with those two dimensions. once drawn add 1/2"all around it for seam allowances. you now have a pattern! Use your extra fabric and cut out this triangle.
When you sew the triangle in place, it's going to poke out a bit and make the sleeve not lie flat on the table, that's okay! this addition should be worn on the inside of your arm so it shouldn't be noticeable, and if it is it only proves you have bicep muscles
you might need to add elastic back in to make it stay put.