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I use ultrabase on my cr10s and am having very good luck with it. In the past with my older printers I always used mirrors as they have to leave the factory optically perfect, meaning, they have to be flat. Mirrors are easily cut as well.
One trick that I have seen used with good success is using tape, paper, etc. under the glass the help the heated bed become level. There are several videos on youtube that you can hunt down to see how they do it.
On my mirror surfaces to help the print grip I used PVA stick glue. It's cheap and easily available at many stores.
Good luck!!