I know it's a big ask
Ye gods & graces that would stink for the first few firings . Every cat within a five mile radius will be coming to see this mega midden not to mention the councils " smell investigators "
There is a free online version of it under that name . You might need to make a clock face turntable so you can photograph it at /in three or four heights at 10 min intervals to get enough data for a good print .
Also poke half a dozen or so very straight 2 mm or so wires top to bottom in the sand several place where there is just sand to also allow expanding hot moist air to escape. as the casting medium gets heated via the molten metal being poured in .., There is no problem in running out half a dozen air vent channels from the casting void and gently removing any crumbs to give you unrestrucet venting awaay from thje cast .
guess how I know this 
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I know it's a big ask
Would you send me the ST file or the complete file for the element spiral .
I'm thinking of having the melter hole about six inches in dia and about 8 inches deep and adding a 40 mm walled drain hole in the floor for any accidental disasters to run out into a deep partially sand filled box .
Kiddies kiln dried silver play ( or builders silver ) sand from big stores & builders merchants or like mine 2 x 15 kg bags off eBay is OK but I'd recommend you run it through a fine sieve first to get rid of the 1.5 mm & above grains ( about 8% from what I took out ).Ok, "affordable fine sand" thenI asked, they only do blasting sand.
My thoughts are cast the melter walls over the form , pouring the casting melter's walls around the form whilst it sits in the metal body of the melter . , making sure you have enough casting medium a you'll need to fill the metal body to the top . After a month or so of it sitting in my glasshouse it should have dried enough to invert it on a stand and use my tar burner lance to gently melt the plastic from out from underneath . Once it's reasonably melted out cook it like this on a low flame for a couple of hours before letting it cool and the=n inserting the elements in the now formed lipped spiral .@Kram , ref the comments you and @DAPPH made over on the over thread ref retaining the element etc. Appreciate my lad's PLA former is sitting there ready for the MK2 one day but us there any milage in PVA?
We print a copy of the actual clay mould in PVA. In effect a cylinder with a recessed, hooked groove in its walls. We then fill with hot poured, red casting wax. Soak and let the PVA dissolve. We're then left with a hard wax former for the clay to wrap around.
Just thinking out loud
Is the clay poured or hand moulded?
Not sure when done how easy it'd be to push the element around the spiral?
Adapt the brass so it becomes a unique must have hand cast bottle opener & sell them after costs for charity or your own pocket & on the likes of Itsy ?
It takes ages to get a copper coating to stick and develop any thickness , I wonder if I should have done a prep-coat of zinc or some other metal when I played at it 30 years ago
I'd maybe come see you on the way back with my lad if convenient and drop SWMBO at that big shopping centre.
For the pva, cow mould face down, put a sheet over it with a hole to fill with wax.
The pla plaster mould needs to be horizontal, and will be in the sand box, similar to the brass medal I did - it was filled from below.. something like this. White circle is your plaster mould, red is filling basin/sprue. Yellow cicles are feeder and the lines are the runner/gate below.
