Is there any reason we can't just use the barrel plate from Unicomp/Ms?
In my opinion, the steel front plate of the Model F is the single most significant component for its feel.
Very good point. I've not thought all that much about that. I've always accounted it to the heavier pivot plate and lack of membrane/rubber layers. On an F, the pivot plate only ever touches hard surfaces, so it moves smoothly and without resistance. On an M, there is pressure against the soft and clingy surface of the rubber mat and membrane layers which causes friction on the flipper and makes a heavier, less crisp keypress. Although, now that you've brought it up, I'm sure it's a combination of a lot of things.
The plate will also certainly add weight which will be nice in general to prevent sliding on desks and such. The only problem is that if we use Model M cases, the top two nubs that hold the plate in place will be much more likely to break if ever shipped as a pre-assembled unit. A couple of old Ms I purchased had them broken off already with the M as "light" as it is comparatively. That is, of course, looking too far ahead. We need to have keyboards to exist before we even consider shipping them...
don't fit, but if we are making a new design, might as well make them fit, but the best answer is IBM F original separate barrels so we can do any layout.
Good to know, I suppose. If we make a new design, it would be nice to have them fit an M barrel plate to open the possibility of using one for reduced cost at the loss of a bit of the Model F feel.
And yes, for custom layouts, individual barrels is by far the best method. However, if we can modify the design to accommodate more possibilities without increasing the cost of the part, it will be worth it. It might even lower costs for those who want it. For instance, an Xtant using the custom metal front plate might be $250 or more, but without the labor of cutting and bending the plates, someone could use a Model M for parts and assemble it themselves for $100 in parts (numbers just made up for example). It won't be the superior typing experience, but it will be a lower barrier for entry.
[Edit]: I'm not sure how much effort the plate manufacturing will take/cost. If it's not all that much for someone, somewhere, I'm much less opposed to using a custom plate. My only concern is if the plates alone cost like $75, that's a really big price jump that might prevent some from trying it out.