Fellow redditor _tardigrade_ aka hmsima published 3D printed Alice gap fillers:
The gaps between keys on Alice keyboards have always bothered me. So I designed and 3D printed these blockers to fill the gaps – tardigrade.
These parts friction fit between the switches, so it’s easy to install/remove them by just removing keycaps. They were designed to fit the Owlab Spring PCB (pictured here on an acrylic spring + Owlab spring PCB + plate), and the author is not sure if it’ll fit other boards as snugly.
You should be able to add some tape as needed to fit it in other boards (like the pictured Rosenthal).
As far as Tardigrade can tell, these fillers have a minimal impact on the sound profile and do not interfere with the keycaps.
I'd love to see how it fits on other boards since I only have 2 Alice boards to try this on – tardigrade.