I've done convex grinds two ways on my Coote. Originally I would basically flat grind the knife, then use the slack belt area between the platen and idle wheel to grind in a convexity. This worked okay, but it took forever and the belt seemed to move around too much doing it this way, occasionally causing it to grind the side of the machine, too.
Someone gave me the advice of gluing a pad to the platen and basically just "flat grinding" like that, which would naturally produce a convexity. Neoprene with the carbon platen fabric over it would probably be ideal, but I simply used some of that cheap blue camping mattress material that I use to make Kydex sheaths on the platen with double sided tape and this actually worked pretty nicely. Occasionally the belt would grab the material and send it across the shop, so it wasn't the ideal setup, but it worked for occasional use. Again, I would flat grind the majority of the blade, then tape my pad on the platen, then finish it up like that. Pretty basic, but it worked nicely and it was a great piece of advice.
