Doodler,
You do know that reads like your asking for modelling advice from a six year old😉
No joking aside this foam clay is marketed as being used by kids to completely cover forms or household object and toys.
It doesn't need additional adhesive ,as its in a sort of pva snot.Just pull a little bit our of the tub, stick it in the box,prod it into a shape your happy with,leave it over night(says 24hrs on the tub) job done.
You'll achieve a stronger bond on to raw mdf,and card . As they absorb a little of the adhesive in the foam clay.
Shahbahraz, I like the over all feel you've achieved,and I especially like the addition of the well. As adds to the 'story telling' of the piece.
If I may suggest the addition of a wooden cover on the inside of the will . A simple dick of balsa scored to look as if its constructed of planks with(or without) a small square in the middle to represent a hatch.
Just that extra zone of colour all be it old wood will lift your work even further.
Plus it lends its self to being more than a well (once the centre is hidden.) a hidden entrance for example.(Three kings springs to mind.).
Another option would be to add a simple counter weighed bucket hoist. Which could be positioned to subtly support and strengthen the palm tree .
Cod,
I recommend using a combination grout and adhesive. Bit stronger and less likely to crack with age.
The nice touch is it takes subtle washes which will give better depth of colour than a pre coloured grout.
Mark.