One of the nice features of Bitcoin is that it's supposed to retain its value better than fiat currencies, which are designed to lose value over time.

Keeping that in mind, you could put a non-spacing unicode equals sign under the B like this: B͇. Looks quite nice also.

If you wanted to keep the double slashed dollar/euro/yen theme, you could also use non-spacing unicode to make something like B⃦ or B⃒.