Ten years of wearing sensors here (Libre since the first version, Dexcom in rotation) and knocked-off sensors were my biggest source of waste until I started overpatching every single one. Any of the stretchy patches sold on Amazon as Libre or Dexcom patches will do the job, they are mostly the same kinesiology tape material with a hole for the sensor, and the generic ones work as well as the branded ones. The cheaper route is a roll of kinesiology tape and scissors, which is what I do these days. Also worth asking Dexcom support before buying anything, in many regions they send overpatches for free if you ask. Two small things that help as much as the tape: apply it to clean dry skin and give the adhesive a day before swimming, and placement matters, slightly toward the back of the arm catches on door frames a lot less than the outer edge.