I used 25mm wide Gorilla tape in that woven black finish that looks like Carbon fibre. I put one layer down to the left and one layer down to the right. This fills the bed and ensures a double layer over the spoke holes. The double layer stops the air pressure from depressing the tape into the spoke holes. That is important to ensure that when you are inflating the tyre tubeless, the incoming air can't escape between the tyre bead and the depressions (if that is where the beads naturally lie). Ensure that each run of tape starts an inch or so before the valve hole. Punch through the valve hole with a nail or a bradawl from the tape side. Then open it out firmly with something tapered. This should pretty much guarantee that the tubeless valve will seal.
Note: There are many fans of Gorilla Tape on this Forum and elsewhere (my LBS for ex). But some on this Forum have said not to use Gorilla tape because it leaves a sticky residue if you need to remove it. OK, fair comment. But I have never had to remove any I have applied, and if I did, there are all sorts of proprietary stuff that will shift sticky residue: meths, Iso-Propyl Alcohol (brake cleaner), petrol, etc. My wife has a product from Lakeland called "sticky stuff remover" that shifts anything! Heaven knows what's in that!
Note 2 added 11th Feb'21: Do not use Gorilla tape! It is the spawn of Satan! I entertained the Forum for a week with my Gorilla tape removal woes. The first problem was removing the tyre! Long story short: If you have used Gorilla tape and need to remove it, strip off the outer layer and remove the adhesive gunk with a rag soaked in petrol. Keep rubbing and suddenly it will come off! Do it outside!!!!
