I recently completed the 'ruined portal'. It's a fun piece of terrain begging for a scenario involving an escape, or disgorging interdimensional nasties!
The 'lid' is removable, allowing for a phone playing a suitable video to be slotted inside.
Before painting I used baking soda dusted over a thin layer PVA glue, to obscure the print lines. Black gesso and brown / grey Halfords spray paint next, finished with 'dirty down moss' for the recesses and a few patches of flock and grass. The dirty down moss is a dream to use: the rust and moss are going to remain in my toolbox permanently. The stuff can be reactivated with a little water on a paintbrush and nudged until you're happy with it.

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