Gradual Media Player Volume Reduction

As I close my bathroom cabinet I want the Sonos to fade out. I've tried to do a rule that repeats 'Lower volume' every 2 seconds for 10 repeats, however Sonos doesn't seem to accept that command. You can set 50 as 50% but not raise/lower it seems.

One way round this would be to set 10 rules, with incrementing delays with reducing percentages, e.g.

Reduce to 40% (delayed for 2 seconds)
Reduce to 30% (delayed for 3 seconds)
Reduce to 20% (delayed for 4 seconds)

But this wouldn't be very smooth unless I had about 20 of these.

There must be a more elegant way?