Finally, the lid for the battery compartment is available for the Olympus OM-D E-M1 II. You may order it as spare part at via davinci, item number VP924500. Currently, delivery time is said to be 4 to 6 weeks.
With this part, if properly modified, you may use the BLH-1 battery adapter with the battery compartment closed. I ordered one and will write a modification guideline.