The keyball and trackball manuform variants[0] all seem great to me. My one issue with them is that the thumb clusters are not symmetrical.
I don't have money for a svalboard[1] so I opted for an interesting variant found on AliExpress[2]. Unfortunately the delivery person threw the package on the ground[3], so my integrated pointing device project is in limbo until this curious planck-like keyboard arrives[4].
Happy Charybdis user here. If you ask, I'm sure Quentin would be willing to build a Charybdis with a trackball on both sides - he was very accommodating of a change I requested (I wanted ball transfer units). He already supports having the trackball on either side.