The sandboxes are glued into the shaped recesses at front and back of the sideframes.
These are the same shape so it doesn't matter which side they go on.
front sand boxes
Took a bit of thought to make sure I got these the right way round.
You can see the small cap on the top of the sandbox here. In my over-enthusiasm I filed the top of the back boxes flat. I'll have to represent their caps with a couple of strips of something.