To play Thud, whether regular rules or Koom Valley rules, you need a 15x15 board, made round by lopping off the corners such that the straight edges are 5 long, and the diagonals are 6 long. You then need 32 "dwarf" pieces, 8 "troll" pieces and one central "Rock" piece.
The board dimensions are critical, as is the number of pieces. Any other board size, any other number of pieces, will result in a giant advantage for one side or the other. The balance is all about the timing it takes for trolls to reach the dwarves and start splatting: it has to be exactly the same amount of time as it takes a skilled dwarf player to have the start of a block built, so they can start hitting trolls in return. And that means you need five squares clear between trolls and dwarfs at the start. Four squares, and it's a clear win for trolls. Six squares, and dwarfs wipe the board. Remove the rock in the middle, and it's a clear win for trolls. And so on. It's not like go where the board size can be changed within reason without affecting the core gameplay: any change to the board changes the balance.
It might be easier to just get cardboard and draw the board out, or to get a decent sized go board. It's certainly not immoral to play the game with stuff you made yourself, nor stuff you bought elsewhere and co-opted. Though I just checked and it looks like Bernard "the cunning artificer" Pearson's Discworld Emporium of Wincanton (the only town in the world officially Twinned with Ankh Morpork) does still sell boards and pieces of great beauty.