The Convention Emergency Kit
Every cosplayer should carry a small repair kit at conventions: fashion tape, safety pins (multiple sizes), a travel sewing kit with thread matching your main costume colours, small scissors, a spare set of silicone cups if you're using them, and a small bottle of fabric glue.
Split Seam
Safety pins every 2cm along the split seam will hold it closed for the rest of the day. For a more permanent fix, hand-stitch with a whip stitch — 6 stitches from your travel kit will hold better than you'd expect.
Strap Failure
If a strap breaks at the attachment point, a safety pin through the strap and costume fabric is a reliable emergency fix. Pin from the inside if possible so it's not visible. Multiple pins in a star pattern hold more securely than a single pin.
Bra Showing
Fashion tape along the edge of any visible bra instantly secures it to the costume fabric. For cups that have come loose, re-stick with the adhesive from a spare silicone cup or use body tape.
Expert Practitioner
This Guide Is Informed by Heidi of Chimera Costumes
Heidi is a master seamstress who builds every costume herself to fit a large bust. Her free content on Twitch, YouTube, and Instagram @ChimeraCostumes shows every technique covered here in practice. Commissions available via ChimeraCostumes.com. Adult content on Patreon and OnlyFans (18+).