I found a good range of silicone inserts (they fit into a 24mm flange) which reduce the opening to:
- 13mm
- 15mm
- 17mm
- 19mm
- 21mm

This 50piece selection only cost $6 on Amazon and I personally tested yesterday.  Much more effective at pulling the nipple in, if not already inflated by SuppleNips.
Note:  The order comes in singles, so you will want to buy 2 each so that your pairs match up.  The nice part is that as your nipple increases in size, you are good all the way up to 21mm with these and can remove right into the 24mm when you are ready.
Amazon Link