If you ever see anything french which looks interesting I might be able to give a better english translation then anything online... unless you find my english incoherent then it would be pointless.
Basically you would not want the sum of the hole areas to be less then 60% of the carb cross section area. The stock exhause you are more concerned about derestriction then reflection of the shock wave to give you that wonderful reversion shot to over fill your combustion chamber. There is not much tunable about the exhause length, chamber size or bulkhead location on the blunderbuss.
For sound deadening you would want many small holes to make up the 60%, if you are unconcerned about the sound then just get some goodly holes in the bulkhead.
Focus on the size of the final outlet by experimenting with a block off plate - somewhere around 11mm or 7/16 inch is a goodly size, if you have a 12mm carb. (edited)