Don't put your finger in those holes. Trust me it is going to hurt. If you are doing 1 piece the boomerang is your best bet but it will take a while to get them from Prince and as I understand it you may have to call a few time to get them. If you are doing 2 piece I like the two piece 50/50 method. Both methods are in my video.
There are other option some people use a sharpie cap which is about 1/2" in diameter. I don't like them they are angled and can fly out. Get a wooden dowel or anything else that is about as round as the thickness of the distance between your crosses. There are only going to be about 4 or 5 crosses that are effected by the. Those are the first ones below the normal grommet holes until you get to the center of the racket. Once you get to the center the normal pull of the tensioner puts the string in the bottom of the port and all is ok. Once you find the spacer you want place it perpendicular to the string bed, close to the frame, between the string you are tensioning and the string above it while pulling tension. Clamp your string and move on.
Here is anther trick that may help. Some stringers have adjustments for the 6 and 12 o'clock supports that adjust separately. If your's is like that mount the racket so it is off center and the swivel for the bar is as close to the top of the racket as possible. I am not a big fan of this option but it was one of the USRSA's top tips a while back so I doubt it is bad for the frame.
Then there is the brake and hip methods. The brake holds the racket in position so the strings stay in the bottom of the grommet where they need to be for clamps. Twisting the stringer and holding it with your hip works too. If you have a table top with no brake neither of these work.
Best of luck