Since you are in training, go ahead and cover the entire ball with leukotape. Make sure your skin is dry. Bring the tape all the way around to the top of your foot. Use scissors to round the corners of each end of each piece. The only time I've had leukotape come off my skin is when I wanted it off, or when my skin was damp in the field. However, it just lifted at the corners and did not pull off the major surface area. I also think the adhesion issue was due to placing the tape on a pencil so I didn't have to take the full roll. That appeared to lessen the stickiness when "applied" a second time to my feet.
Everyone that I have known who uses plastics tapes up. Just as you noted, unless they have a heat mouldable inner boot, you have limited options for fitting them to your foot.