Mabroum has made a few boards with this sorta layout. They've opened sourced them and can be found at https://github.com/AlaaSaadAbdo/battoota
I've been using thumb shift since day one and I don't see myself changing that.
I completely forgot I had a ks27 KLOR to build....
If you're comfortable around a soldering iron, building a split can be pretty cheap.
What sorta features are you looking for? How many keys?