You'd be better to buy a sheet of the Baader solar film and make a filter from that.
It's around $35 for an A4 sheet and comes with instructions on how to make a good filter. I used it with my 300/2.8 and 2X teleconverter to shoot this pic.
https://www.flickr.com/photos/pw-pix/12377076214
https://www.flickr.com/photos/pw-pix/12377061134