The knit cast on or the cable cast on would work for this as it keeps the yarn in the right place no matter how many stitches you need to cast on.
The cable cast on has a nice edge, attractive and sturdy but a little bulkier, the knit cast on is less bulky and still neat.
Maybe try them both on a swatch or you can try them on your project and see how they look and which you prefer.
There are video tutorials that can help.
The knit cast on is like working a stitch knit wise but instead of sliding the old stitch off the left needle you leave it on and add the new stitch from the right needle onto the left needle to create a new stitch. And repeat for the number of stitches.
The cable cast on differs by placing the right needle between the first 2 stitches instead of into the first stitch, then work it the same way, drawing the yarn through like a knit stutch and adding it to the left needle.
On the videos they are starting fresh with a slip knot, you don’t need to do this just start with the work on your needles and use your current stitches and working yarn.
Hope this helps, if you need more help please say.