You and I have obviousy been thinking along the same lines! I thought the battery on top of the rack would scream "ebike" and wanted to disguise it. I wanted a rack top bag to enclose it - even to the extent of cutting the bottom out and just sliding it on top, but I couldn't find one long enough.I have a couple of pannier bags here that sit on the top of the rack only. I was hoping one of them would be large enough to accommodate the battery. I would have fitted some internal padding to protect it in case of the bike tipping over so in the meantime I am going to keep an eye out for something like that. With a pannier bag under the battery and draped down each side, it would look very discreet indeed I suppose.
AJ
In the end I bought a battery cover off ebay:-
Bike Battery Protective Cover Water Proof Bag Ebike Battery's Battery Cover | eBay
I think it's a different size to yours but there are lots of different ones on ebay.
Ok, it's still battery-shaped, but combined with a pair of roll-top pannier bags it's a reasonable disguise, particularly with a light waterproof jacket strapped on top.