I just used whatever I found around the house.

I did not happen to have proper double-sided tape, so to keep the battery back in place, I used one pre-cut to small squares (about 8x8 mm), designed to stick pictures in frames. Similar to this one. I used four squares, one in each corner. It holds fine.

I did not bother with the backplate at all. Whatever sticky magic they used still holds even after a second battery replacement. It is possible that my usage pattern (keeping it in a flip case and playing an Android game that can get the phone quite hot) helps