Having designed everything in Fusion 360, it was easy to make line work for all the parts of the mold. The mold is basically an 1/8" thick acrylic copy of the burner that is 1/16" thicker than the burner itself. This allows for an offset to give you some wiggle room to put the burner in.
I used acrylic for the positive mold because it's rigid and makes for a very smooth cast concrete surface that doesn't require grinding or machining. With acrylic cut to size and welded with plastiweld, it's easy to get solid, air tight seals on all the parts.
Tap Plastics has a great tutorial on how to use acrylic cement to weld acrylic sheet together. For this mold, I cut the parts to size using the layout attached in this step, then built the box so that it was enclosed on all sides. Having no plans to re-use this mold, I decided to make it one monolithic piece, then break it apart with a rubber hammer and putty knife later.
I sealed the inside of the cutout on the table top with several coats of polyurethane to keep the water from wicking into the wood and making it swell.
Next, I aligned the cutout on the table top with the mold positive- this was easy to line up since I cut the bottom surface of the mold to the same size as the table top. I clamped the top down with more clamps than I thought I needed to get a tight seal.
Then, I mixed the Pro Formula into the concrete mix. I used 7LB of concrete mix, because I calculated that would fill the space about half way, which would be plenty. After stirring with a paint stirrer to get all the dry clumps out, I poured the mix into the cutout and let it cure over the weekend. 1 bag of Pro Formula mixes into 120 LB of concrete mix, so you have to do the math an measure the right amount for a smaller volume of mix. I have enough Pro Formula leftover for 20 more concrete cast projects after this.