a good calculator for this can be found here http://homedistiller.org/calcs/dilute
Put coke bourbon and keg in a fridge set at 0c, this will help keep CO2 in solution. then just fill the keg, purge and set to serve pressure. i have found unless you have time dont **** with the CO2 volumes.
If you are going to have something fabricated you are better of getting what you want. It costs money to have things custom made and as cheap as he may be it is still going to take coin to buy materials and consumables. things like argon filler rod grinding discs they start to add up.