That plate looks great! If you've got any feedback on it, I'd love to hear.
For a computer mount, I've had a few solutions.
The first one was a 3D printed plate that was zip-tied to the 2 extensions and had a Garmin insert bolted the middle. This got the computer right between my wrists.
Now, I made a much longer bottle rail and a 3D printed mount on the rail with the Garmin insert to attach the computer to.
Note that the rail you have pictured is meant to be supported by a frame underneath (it is used to move bottle mounts up/down a frame), and may move too much and break when hanging a fully loaded bottle off the end like that. The one I designed is much thicker to properly handle the loads and not have a bottle shake loose. I'd also recommend a 'saddle' cage for the upside-down cage where it has the hooks opposite the screws. I use a Topeak one since its the cheapest I found, but there are a few carbon cage options as well.
Happy to share the files if you're interested. I'm also setting up a site to sell some of my custom parts.
I just saw your reply. I'm using the botlerail-extender thing as wel, and I'm a little bit concerned about durability as well. Mine doesn't have a long 'overhang' and bottlecages are mounted on top and close to the BTA riser/spacer --> I think I'm good, but your rail looks really nice, especialliy with the option for the Garmin mount (min is between than hands, with the risk of accidentally pressing buttons).
Is it possible to order that rail (and what would be the costs)?