I found some existing cases that may work for the FunKey S (check image below). It is difficult to test these ideas before we get the product, unless someone is willing to 3D print a block that has the same dimensions as the FunKey S to try some cases. I'll leave the Aliexpress links for these cases below the image.
Airpods / Airpods Pro cases (with clips, zipper, or magnetic clip)
Airpods silicone sleeves (one-piece, not the two-piece ones)
Other silicone sleeves
Smartwatch silicone bumpers (if they stretch enough)
Coin purses / earphones cases / SD card holders (some may big, though)

Airpods cases that clip shut:
1. https://www.aliexpress.com/item/4001027619781.html?spm=a2g0s.8937460.0.0.6e3a2e0eoW2cKd 2. https://www.aliexpress.com/item/4000500417333.html?spm=a2g0s.8937460.0.0.6e3a2e0eoW2cKd 3. https://www.aliexpress.com/item/4000412112143.html?spm=a2g0s.8937460.0.0.6e3a2e0eoW2cKd 4. https://www.aliexpress.com/item/4000008375750.html?spm=a2g0s.8937460.0.0.6e3a2e0eoW2cKd 5. https://www.aliexpress.com/item/32906960345.html?spm=a2g0s.8937460.0.0.3a6c2e0exl6giT 6. https://www.aliexpress.com/item/32977946291.html?spm=a2g0s.8937460.0.0.78dd2e0eiKzATa 7. https://www.aliexpress.com/item/33016835358.html?spm=a2g0s.8937460.0.0.78dd2e0eiKzATa 8. https://www.aliexpress.com/item/32978291841.html?spm=a2g0s.8937460.0.0.98702e0eCgB5Op 9. https://www.aliexpress.com/item/4000398907965.html?spm=a2g0s.8937460.0.0.98702e0eCgB5Op 10. https://www.aliexpress.com/item/4000460680938.html?spm=a2g0s.8937460.0.0.74702e0emVZ1F6 11. https://www.aliexpress.com/item/4000470231841.html?spm=a2g0s.8937460.0.0.74702e0emVZ1F6
Cases with zipper:
2. https://www.aliexpress.com/item/4000632988358.html?spm=a2g0s.8937460.0.0.98702e0eCgB5Op
3. https://www.aliexpress.com/item/4000065671883.html?spm=a2g0s.8937460.0.0.98702e0eCgB5Op
4. https://www.aliexpress.com/item/4000507472606.html?spm=a2g0s.8937460.0.0.74702e0emVZ1F6
5. https://www.aliexpress.com/item/32964425993.html?spm=a2g0s.8937460.0.0.4ee32e0eJrDQg8
6. https://www.aliexpress.com/item/33028411511.html?spm=a2g0s.8937460.0.0.7c922e0eQYefge
7. https://www.aliexpress.com/item/32911408599.html?spm=a2g0s.8937460.0.0.7c922e0eQYefge 8. https://www.aliexpress.com/item/4000760389761.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 9. https://www.aliexpress.com/item/32827459997.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 10. https://www.aliexpress.com/item/4000525266699.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 11. https://www.aliexpress.com/item/33048262394.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 12. https://www.aliexpress.com/item/32968385628.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 13. https://www.aliexpress.com/item/33000815099.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 14. https://www.aliexpress.com/item/33004297035.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 15. https://www.aliexpress.com/item/32858106773.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 16. https://www.aliexpress.com/item/32857212157.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14 17. https://www.aliexpress.com/item/32788744547.html?spm=a2g0s.8937460.0.0.c67c2e0e1e4y14
Silicone sleeves: 1. https://www.aliexpress.com/item/4000538019801.html?spm=a2g0s.8937460.0.0.4ee32e0eJrDQg8
2. https://www.aliexpress.com/item/33012738135.html?spm=a2g0s.8937460.0.0.7c922e0eQYefge 3. https://www.aliexpress.com/item/32974881518.html?spm=a2g0s.8937460.0.0.7c922e0eQYefge 4. https://www.aliexpress.com/item/4000080375331.html?spm=a2g0s.8937460.0.0.7c922e0eQYefge 5. https://www.aliexpress.com/item/4000470422548.html?spm=a2g0s.8937460.0.0.7c922e0eQYefge
Smartwatch silicone bumpers: 1. https://www.aliexpress.com/item/4001136366015.html?spm=a2g0s.8937460.0.0.6cb62e0eHgqsCI 2. https://www.aliexpress.com/item/4000429006122.html?spm=a2g0s.8937460.0.0.6cb62e0eHgqsCI
Other Airpods cases (may not stay shut): 1. https://www.aliexpress.com/item/4000666638811.html?spm=a2g0s.8937460.0.0.74702e0emVZ1F6
Cool Airpods silicone cases (may only be usable if someone can design and 3D print an adapter piece): 1. https://www.aliexpress.com/item/4000881278648.html?spm=a2g0s.8937460.0.0.3e352e0esF74VW 2. https://www.aliexpress.com/item/4001110370670.html?spm=a2g0o.detail.100009.4.79224ceaPPQXd6&gps-id=pcDetailLeftTopSell&scm=1007.13482.95643.0&scm_id=1007.13482.95643.0&scm-url=1007.13482.95643.0&pvid=0d0d15c3-74fb-42d6-a321-ae3ce6bf51af&_t=gps-id:pcDetailLeftTopSell,scm-url:1007.13482.95643.0,pvid:0d0d15c3-74fb-42d6-a321-ae3ce6bf51af,tpp_buckets:668%230%23131923%2358_668%23808%233772%23997_668%23888%233325%232_668%232846%238109%23239_668%232717%237564%23631_ 3. https://www.aliexpress.com/item/4000008229663.html?spm=a2g0o.productlist.0.0.210822062QWp2d&algo_pvid=898e512d-ad5d-4344-b2d5-4dd54995218f&algo_expid=898e512d-ad5d-4344-b2d5-4dd54995218f-4&btsid=0ab50f6215941705510891648eb1c3&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_ 4. https://www.aliexpress.com/item/4000171755170.html?spm=a2g0o.productlist.0.0.210822062QWp2d&algo_pvid=898e512d-ad5d-4344-b2d5-4dd54995218f&algo_expid=898e512d-ad5d-4344-b2d5-4dd54995218f-5&btsid=0ab50f6215941705510891648eb1c3&ws_ab_test=searchweb0_0,searchweb201602_,searchweb201603_
Feel free to comment on these ideas, and add your own.
Amazing list Douglas!