Solution for ND-Filters on Canon's 11-24 F4/Canon 8-15 F4 Fisheye

I am not sure, if this is the right forum, but I am sure people who are interested, might like to hear. I do a lot of long exposure photography, and I work with various lenses and filters. I have the Lee System, a few B&W's ND10 and most recently purchased of few glorious Neutral Density Filters from Breakthrough Photography. I would recommend everyone these filters, as they are just the best. Anyway, back to what I am trying to share with you. I am thinking about purchasing Canon's 11-24, but one very crucial element is how I could attach filters to that. I think it was even featured here on Dpreview, that Fotodiox have developed an adapter to attach Filters to that particular lens. I had a closer look at that Dpreview post, and wow what a machine of filter attachment that is. It really made me think twice if that is what I want to do. I started looking at alternatives. You might be aware or you might not be, but the 11-24 as well as the 8-15 F4 have a slot for a gel filter in the back. When I started looking into the use of gel filters, all I could find, is posts from people who used a filter sample book from Lee and and then cut those gels to size. These are generally coloured gels, not ND's. Alternatively there are sheet gels used by film makers, but here the issue is that they are only available in 1, 2 and maximum 3 stops (0.3, 0.6, 0.9). Not a solution for me. I am looking for really strong ND's.

There are films available from Kodak (Wratten 2), available as Neutral Density in all stops, including 2, 3, 6, 10 and 13 stops, but they come with a price tag. I think around $200 per gel sheet in 4x4". I didn't know that, and they are even available at B&H, however I found an even better solution. I found this company in the UK, they sell high end filters for medical and scientific applications.(knightoptical.co.uk) Small sheets of 50mm x 50mm Neutral Density gels in 3,6,10 and 13 stops. 25GBP each. I couldn't resist and ordered one of each. I can't wait for them to arrive. Once cut to size, I will stick them into my fisheye. I can't wait to see if this works. Daytime long exposures I am sure are not everyone's cup of tea, but if that is what you are interested in, I think this could be an actual solution. The only concern I have is probably light leakage. The light enters the lens and then all the way down until it hits the ND film. It might be that I end up with a leak down there. No matter what, I am so excited, I give it a try. I haven't found anything on the net about this. If this really works on my Fisheye, I am up for some crazy unique shots. Will share them here, so you get to see the results. And if that really works, you might have an actual solution for your 11-24.

If you have you done something similiar, I would love to hear from you.