Re: Homemade center filter
Try smoking a filter over a candle.
Re: Homemade center filter
I included you, man
but I'm not supposed to play with fire
Re: Homemade center filter
I once made an experimental centre-dark filter from two 75mm diameter spectacle lenses, a +2dioptre and a -2 dioptre. The +2 was dyed dark brown for use in sunglasses. Held together the powers cancel but the centre-dark effect was perfect. Total cost was $50 and the optical quality was ok. Colour neutral? No way!
Re: Homemade center filter
You could try dyeing a CR39 resin filter.
Its not that hard to gradient tint an eyeglass lens, iirc 50% took about a half hour and we had to move it every minute or so to keep it even enough. IIRC (its been years) there was also a bleach that would remove most but not all of the dye. The stuff is used hot, about 180f if I remember right. I believe Cokin uses CR39 as the resin for their filters. Don't expect true color neutrality, there's a reason these things cost...
Not sure where you'd source the dye but any one-hour eyeglass place would have the stuff and you might talk one into giving you their old stuff when they change it.