The easiest way i ever found to do it in rally cars ,was to get a heavy duty bin bag,the cheapy ones split, open it up,to prevent it sticking,then place it in the seat,sealed end to the bottom. Having previously bought your can of high expansion PU foam,available at any builders merchant for about £4,attach the nozzle invert the can put the nozzle down the side of your body between the seat inner and yourself,but obviously INSIDE the bin bag,spray, repeat on the other side. Bear in mind it expands to at least 4 times it's volume so don't overdo it. Next,grab your ready made flask and newspaper/radio/tv whatever and allow to harden for a while ,best part of an hour or so at least. If you want the foam quite firm seal the bag before it starts to ooze out this compacts the foam and makes it denser.Obviously you will need assistance to do that.
That will make a perfect mould that will last for ages ,you can customise it whichever way you wish,but if you cut it before it is fully cured inside,anywhere exposed to atmosphere will expand again. (And it's messy! ) If you want to remove the plastic after moulding use a releasing agent on the plastic before you squirt the foam in,washing up liquid,cheapy not fairy, or spray inside the bag with WD40 etc
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