I received an email from a customer that reminded me of another option for the cups of a swimsuit! So I thought I would share it here as well.
A lot of RTW swimwear and activewear garments are created so that foam cups are removable from the garment. It’s a practical application for many reasons, including reducing sewing time in production as you don’t have an extra step of securing the cups into the garment. From a wear perspective, depending on the quality of the foam they can degrade quicker than the garment itself and also personal preference of the wearer – some of us just don’t want the foam cups! Personally I’ve had some mixed experiences with free floating cups in cami-top style sports bras, where they migrated across my chest during a yoga class. The visual result was highly amusing – I had one perky and one floppy boob!
Back to the cup lining alteration, below is an example of the 8101 swimsuit pattern from one of the original prototypes. I was trying out different options including a partial cup lining to allow for easy insertion and removal of swim cups.
You can see that the lining is cut shorter by about 3-4cm’s than the outer cup, with the top edge left raw and loose leaving only the sides and bottom edge of the cup sewn to the rest of the swimsuit. To recreate this opening, just cut across the top of the cup lining pattern piece, parallel tot he top edge.
That small gap (where the pen is) is just wide enough to roll up and slot in a swim cup, but not too big that it’ll slip out when you’re busy with practicing your latest swim choreography down at the pool.

I almost want to say I’m sorry for finding this gif…. but i’m not.

I’ve seen some low-impact sports bras that have the gap on the lower outer edge instead, so if you want to retain the clean finish around the top edge and cover the strap ends you could just cut out a small wedge on the lower outer corner instead.
In the end I decided to go for a full cup lining for the pattern as when we’re sewing for ourselves it’s easier to cut away from a pattern piece that the fudge around and recreate a shape, and also a full lining creates a cleaner finish on the inside.