I agree with DarrenG. The official name is BMW i Board Bag. It fits the space precisely, has interior dividers sized to hold the L1 charger and tire kit, and rolls closed with a secure velcro closure to keep out water. Biggest criticism is that it's overpriced in the US but I found it for a significant discount by doing a little digging. That was over a year ago and the BMW parts retailer I bought from has raised its price. Still with knowledge that not all BMW outlets charge the same prices, you might find an acceptable deal.
Indeed, I baulked at the list price but found a dealer selling via Amazon here in the UK and got it for near half the list price which is closer to what it's worth IMHO.
The BMW i bag does fit perfectly. It's probably not inexpensive, though. I got mine for free as a perk for getting an Electronaut Edition i3 so I don't know how much they are.