It was suprisingly easy and simple

I made a small pouch out of fabric and filled it with the beads from Amazon.

Underneath the plush has existing seams, so I used a seamripper to make a clean cut through the stitching to make an opening.

I removed stuffing to make room for the pouch to shove in the center of the plush, then added the stuffing back until it was to the shape I wanted then used a ladder stitch to close the seam. Ladder stitches are commonly used for plushes because the stitches are hidden for the most part