I'm afraid that trains are taking over! Here is a custom mineral load for the truck which comes with the 4010 train set:
http://www.playmodb.org/setpics/4010.jpg(Picture from playmoDB)
The load is item 7843 from the DS catalogue:
http://www.playmobil.co.uk/on/demandware.store/Sites-GB-Site/en_GB/Product-Show?pid=7843&cgid=Ds_EisenbahnIt's not ideal for using in a custom load as the pieces tend to be concave which reduces the area for putting glue on, but overall it's usable.
I thought for some time as to what to use for a base and finally came up with the idea of a polystyrene block. This has the disadvantage of flaking badly when it's cut however, making the place look as if it's just snowed!. I decided to see what happened if I first coated it with PVA adhesive which I used without diluting with water.

I coated top and bottom, the sides and the ends and then cut it with this knife which is designed for cutting frozen food:

I used the smoother (lower) side of the blade and sliced rather than sawed away at the polystyrene. Although some bits flaked off, they were very few.
I coated the cut edges with PVA glue and then painted the top surface with Humbrol grey 32 once the PVA was dry:

To stick the load to the polystyrene, I put the cut polystyrene block in the truck, pushing it down so that the top was just below the top of the truck side. (If you gently push it down, it stops when it comes up against the ends of the outer handles which stick out inside the truck. (I'll post a photo of the inside of the truck later.) Then I coated it with PVA glue and scattered the load over it. This is where the size and hollow shape of the pieces can cause problems, but with care I found that most of the surface was covered.
(I'll have to stop here as it's time for lunch and then we're going out to the local miniature railway's 80th anniversary.)