Fixing Window Box to UPVC Window Frame

[mintspy]

Hello,
I'd like to fix a window box to our 2nd floor window. I'm not able to make any permanent fixings due to our lease. We have sliding UPVC windows (sash style) so I'm thinking of making some fittings to fit inside the track that the window slides up and down and using them to support the Window Box (am I making sense?). Most of the weight will be down on the window sill, I just want to make sure it doesn't get blown or knocked off.

I'm just wondering if any one might know if this is a really bad idea? or has tried anything similar?

Thanks,
M