Not my usual design, but my boys are now too old for this stuff, time to maintain it for the next gen. ;-)

Generally, Playmobil has quite a good service and a lot of spare parts for reasonable prices:
https://www.playmobil.co.uk/on/demandware.store/Sites-GB-Site/en_GB/CustomSearch-Spareparts

However, for sets longer not been in production spare parts are not available anymore. One of these cases is the battery cover 30 20 0680 (30200680) of the remote control of some older RC-train sets. So far as I see, this remote control was used in Playmobil sets with electric loco cargo train 4010, steam engine passenger trains 4017, 4019 and 4021 and shunting locomotive 4085.

Hope someone has use of it.