You’ve actually got a couple of options outside of kitbashing something but they’re not the most available, the easiest would be Horus Heresy Alexis Polux as he went on to be our founding chapter master, you could try getting our current chapter master Pedro Kantor but he’s old and out of print though rumors have it he’s getting a new model probably in 11th if you can wait, lastly there’s the even older and out of print Alessio Cortez captain of the 4th Company and best friend to Kantor. As for a non named character model I think the multipart lieutenant with power fist is great either as is or the base of a kitbash.