I've made a USB power interface with a broken extensible mouse cable previously:
And it works so far. Under strong sun light, however, the solar battery will deliver from the solar panel directly and the current is not stable for that case. Also, I found the solar battery might actually drain itself when the ambient light level is insufficient for charging! Therefore, I want an alternative power supply.
For that matter, I've bought a couple of 18650 Lithium battery. When two of them are connected in series, we have 7.4V which is enough to drive the system. At 2400mm, this is more than enough for a whole night of tracking with just two. And I've bought four of them, so it's more than enough!
The USB solution was kept in place.
And the dual 18650 batteries were sneaked at the back of the hand controller.
After testing, one set of batteries (i.e. two of them) could last for over 20 hours. I've four on hand and thus it is sufficient for any purpose.