Interesting project Jo. I reckon you could get the angle in the recess fairly easily using Milliput white and a shaping pice - in fact a the end of a square section of Evergreen would probably do it. Basically just fill the entire recess with the Milliput, then, making sure both the plastic and the surface of the Milliput are well lubricated with saliva (OK - yuk, but who's going to know?) carefully drag the styrene along. It should ride along the edges of the recess squeezing out the excess filler as it goes along and form the angled shape in it. The same priciples work for curved shapes too, just use a round former.
It's easy enough to practice the technique before trying it on the wagon too - just glue down two strips of a backing piece of plastic, fill with filler, then wet and drag. It will probably take a couple of drags, usign a sharp blade to lift off the bulk of the excess from the first run. The smaller amount on the second run can probably be left to harden to be smoothed off after.