Good old foam earplugs are going to give you the best NRR. If you really desire a custom molded piece, I would suggest
https://www.pilotstuff.com/averysound/
You should probably go with deep impression earpieces, which you can't do with a DIY kit. Avery Sound sends you an "order form" which you take to any audiologist and they will do the impressions for you. Good ones will give you a free redo if they don't work, or the earpieces don't fit. Around me, it's about $70 to get the impressions, prices vary, call around.