If you absolutely had to improvise a DIY graduated ND filter to darken bright skies, what would you use? Out of the box thinking welcome! Thanks

It depends on how permanent you want it, but a short term solution would be "lamp black". Put a piece of glass in a candle flame and let the soot black the glass as needed.

Dave

A linear polarizing filter over a circular polarizer filter will work.

Just buy a cheapie filter until you can afford a nice one. They can be had for very little money on ebay.