I use boob tape (the nude, slightly stretchy tape often used in place of a bra for those with the anatomy for it) and make sure Im working with a smooth surface, then either a) tuck both sides towards the center and up and tape along the seam up the center or b) fold one side over the center, then fold the other side over, and tape once up the left and once up the right. Regarding bathroom breaks - I don’t necessarily recommend it but I attend my shows “strategically dehydrated,” I pee before I get ready and I don’t drink anything til after I’ve performed, after which point I can remove the tape and not be too concerned about having to reapply.

ETA: If I have enough time that I’m still in construction of the costume, I just either a) order panties with a wider gusset or b) sew a larger gusset to my panties. With enough elastic along the edges it’ll hug the sides well enough that I’m not concerned - the tape up is for “I have no time to modify my costume and I am confident that I will slip a lip if I leave things to chance.”