good work LR!
I completely understand what you are doing, and how the slots get progressively tighter as you go. It will be very hard at the end.
Off the top of my head, this might work.
With your jig, when you make the first slot (which then gets moved over to locate on the lug) you could always do a second pass thru.
Ie, cut the slot and it is exactly a blade width wide (ever so slightly small). Ok, using paper or shim or whatever works it is in place for that first pass.
Then do a second pass, after removing the shim. The slot is now that shim width wider.
When you move the pack over onto the existing lug, you make the first pass with it (the pack) hard to the left, and then the second pass hard to the right and that way you now have the second slot *ever* so slightly wider than the blade width, and the same size as the first slot....and so on thru the pack.
I too second the safety aspect, when I built mine this way I ended up with a saw cut on my finger!! (see the space coupler theory thread, I too show pictures etc of the cutting jigs
http://www.gearslutz.com/board/4528359-post26.html) I got around that problem by the saw always being 'hidden' within wood, and never exposed. the pics in that other thread may help some.
Can I ask, is there a reason why you did not go deeper?? I know it gets heavier etc etc, but personally for all that work I would have made it 'more' worthwhile and make it diffuse deeper.
You are making more than one?? If so, then with your spacers I would have reversed it. Ie, rather than cut to length the spacers so it 'places' the tiles from 'above', I would have cut the inverse length so that it places the tiles from 'below'.
That way, you can make a full jig that can be used over and over.
The dowels can be permanently glued in place on a backboard, such that your grid can be placed on it. This time when you put the tiles in, you are doing it from the 'back'.
That also now allows you access from the back to the tiles, either for getting the glue in or filling the cavity. Once done, lift off the completed product and place the new grid on and do it all over again.
The way you outlined means every time you want to make one, you need to sort loose dowels and place the spacer manually in the correct location...would not want to get a few out of place!! so lot's of checking etc, then you need to remove the product, turn it over and then do the gluing and filling of the cavity, all the time hoping one or more of the tiles do not move.
Pretty well exactly the same method as you are doing, but without the double handling. just think in reverse.
And only really useful if you are doing more than one.
Not a criticism, hopefully just an alternative look at the method.