The Ka Bin'tak is essentially identical to the Bin'tak only larger and without a top centre prong, (for want of a better word). So simplest proxy is a Bin'Tak with or without its top prong.
The simplest conversion would be to think of the Ka Bin'tak as a command Bin'tak so altering the centre prongs, making then wider/larger would be relatively easy and differentiate the ship from a normal Bin'tak. maybe adding some more weapons to the central structure
Always preferred the prototype Ka Bin'tak sculpt to the final version, (but without the shark fins), especially the weapons cluster, so if I was to scratchbuild one then I would go this route.