Hello Sotiris
I know this Is late and you would have already done the methods above but something I do for most of my hawkmoth larvae is when they start looking for a place to pupate I place each one in a takeaway container full of old porridge oats and as long as these are not wet they will not produce mold, often the larvae will bury down within a couple of hours and then I leave them a week and collect them which reassures that they have hardened (this method is very much like the soil method but much cheaper )
Sam
