This is just a simple idea for physically attaching external speakers
to an electric piano, specifically a Yamaha P95 on its L85 stand.
The P95's internal amp & speakers are great for instant playability as no other gear needs switching on, but you are limited by its internal sounds and small 6-watt speakers - not surprising given this model's position in Yamaha's price range. However, its 88-note weighted keyboard has a great feel and can be used to drive external software or hardware synths/pianos via MIDI... which is where the external speakers come in.
Existing floor or wall-mounted speakers could obviously be used as-is, but if you physically attach them to the piano, they're always in the right place and the vibrations fed back to your fingertips seem to give a richer playing experience.
I found some ex-HiFi 30-watt passive speakers which left plenty of knee
room when placed either side of the stand, but they needed raising off
the floor to avoid me sticking my feet through the cones! They had seen
better days so I didn't mind screwing a couple of bent L-shaped brackets
to the back of each speaker cabinet...
... this means there's no need to drill or damage the piano stand as
the speakers are just hung over the top horizontal bar - this
is the rear view of the left-hand speaker...
If you don't want to screw brackets to your speakers, you could try larger brackets over the bottom horizontal bar to act as platforms for the speakers.
This is the front view with both speakers in place; no accidents so
far, but protective grills might be a good idea if you have them! The
power amplifier is out of shot...
The internal speakers can still be used as normal via the P95's volume control and/or switched via the 'local control' function... which on the P95 is toggled on/off by pressing & holding both the Metronome & Demo buttons before pressing the 2nd highest C key.
...but the audio improvement is worth it.