Carlob made a very nice dessicant box for the Ikea Samla. However some dessicant qualities pebbles are so small they run through the bottom grid. Here is an isert tray to these boxes to allow usage with small dessicant pebble qualities.

I also reduced the volume for the dessicant slightly because I feel thet it is a rather large amount of dessicant and its quite expensive.

Link to the designer files to which this insert fits:

https://www.thingiverse.com/thing:2982939