i am aware this is a very old thread. However, today I came across the news, that Irix [swiss based lens maker] will soon be offering ND gelatin filters - sized 29x29mm and with pre-cut corners to match the rear filter holders on Irix 15mm f/2.4 and Irix 11mm f/4 lenses.
Of special interest is the low price: € 8,45 incl. tax for a complete set of ND4, ND8, ND 16 - 5 pieces each
https://www.ephotozine.com/article/irix-launch-edge-gelatin-filter-set--31087
I was wondering, whether these filters may also fit into Canon EF 11-24 [and/or 17-40, 8-15] rear filter holders ... does anyone have dimensions (mm) of Canon gelatin holders on said lenses?
EDIT: found the info. Apparently Canon EF lenses with rear gelatin filter holder are 31mm. So looks like the Irix ND filters are just a bit too small.
http://www.p4pictures.com/2015/03/how-to-use-a-gel-filter-on-canon-wide-angle-lenses/