Using hot pressed paper stock is the easiest.
Next step up is to use a polymer blend described on the page I put before. It's also useful to give two coats of sizing, but this will pretty much require hardening of the gelatin size, at least for the first coat.
If you want anything smoother than this, the only obvious method is to use baryta sized paper.
I'm not sure if baryta sized paper works well with kallitype. If plain gelatin size works well, I don't see why baryta wouldn't work, but if you want to be sure, you can look up the alt-photo archive, ask a q there, or otherwise ask other kallitypists.