Nigel,
Problem is not really with the distance (though your spacers are a great idea), but the low angle the shutter release lever is currently set at. I would have expected it to have been higher, the lever set level with the top of the housing. But notice on the Ikelite website that the D300 housing is the same. My previous housing for the D50 had a very nicely positioned shutter release that allowed very smooth operation of the shutter. It may be something I will get use to, so will give it a few more goes.
I upgraded from a D70 to a D300 last year and the shutter lever did take a bit of getting used to. I don't really think about it now and can easily squeeze it enough to focus. I don't like the positioning of the sub-command dial near the shutter leaver, though, and for my first few dives ended up firing off shots whenever I changed the aperture.
While you are busy building additions to the housing, have you though about improvements to the 2 button port locking system? I have thought about a ring clamp that goes around the port and locks to the housing some how, any suggestions? I just wish Ikelite would add another port lock, I am sure it would increase sales.
The port locks haven't worried me to date (although I know a lot of people that don't like them).
The thing that is worrying me right now is the sub-command dial control lets water into the housing if pushed slightly forward (which can easily happen if you get hit by surge, etc. while adjusting the aperture). It happened to me at 18m last Saturday. Fortunately, there wasn't a lot of water, the leak detector started flashing and the sanitary napkin took up most of the water. I did a test dive on Monday (Happy Birthday Liz) with just the housing and was able to reproduce the issue without pushing very hard. At this point I don't know if this is a design flaw or something wrong with my housing. It is quite possible that my housing has been that way since last year.