I have done everthing possible to make conkers harder, I promise you, you will never make them harder, if you do make them harder they will just become more brittle and shatter.
I remember doing this experiment at school . Suprised i can remember that far back LOL. Anyway the conker that always came out the best was the one that was left alone. All my mates had these wonderfull ways that never worked like baking them , soaking them in vingar , varnishing them & all sorts of things. But best to just leave em alone. Mother nature knows best, and she takes some beating.
I remember trying every trick in the book and none of them seemed to work, I got my toe's stamped on a few times if I was suspected of cheating or nobling.
I think the best way though is to get up really early in the morning to find the biggest and heaviest ones under the trees (before anyone else is around) I liked the ones with the flat sides, we called them 'cheese cutters' they were a harder target to hit and I remember some kids saying "they aint allowed, they've got to be round ones"
I had a 79er once! Good Luck Tbird+