You pretty much hit the nail on the head, it is your imagination, your Will and your intention which forms the seed or core of the thoughtform. The energy source is entirely up to you.
There are loads of books out there on or involving thoughtform creation. The standard “energy ball” or “psi ball” is a thoughtform or a construct. Check out stuff on Psion and Psionics.
Is a cracking book which was released on a private and limited run, way back in 1991 by my old friend Phil Hine.
In my youth I played around with evoking sound elementals/ sonic thoughtforms. I also explored using musical genres and sounds to as food sources for thoughtforms. It was fun and pretty effective. The idea to evoke sound elementals was inspired by a trilogy by Melissa Scott, whose protagonist was a starship pilot called “Silence Leigh” who becomes a mage. A magic circle was a sonic construct, and the curious sonic hermetic magic really inspired my teenage self.