Ok Atto here's what you can do:
The foam is indeed too sticky and its better not to place it directly on a surface, unless you are absolutely sure you want it to stay there. So the idea of spraying it direclty on steck parts is out of the question. Even if you remove it, it will leave stains on the plastic...
I'd suggest, if you want to make a slope, hill or ridge, make sure you spray in layers and leave each layer to get crusty, but not too crusty, before you spray the next layer, just to avoid the slimy cave effect I got, which will reveal itself when cutting the foam, which btw is very easy to do so if the foam is dry solid...
Once you finish the spraying, and the foam is crusty but not too crusty, you can gently place the steck parts on the surface you want. It won't be solid yet, but it will be crusty thus not allowing it to stick on the plastic. The foam is elastic so it will just touch the steck parts and expands toward the other end, thus taking the shape of the part towards which is up ahead...
So, to sum up,
1. spray in layers, and leave each layer to dry a bit
2. dont let it get too crusty
3. it can be cut easily once it solidifies
4. before it solidifies its elastic and not sticky, so its easy to press something gently up against it
5. make sure you wear gloves
Hope this helps Atto! If you still have questions don't hesitate to ask. I am looking forward to your results!