Sorry for the crap photos - I just wanted to share these extra keys I added to my Lily58.
I noticed there was a gap in the matrix so I tacked a spare hot-swap socket to the pads on the back of the PCB. That worked with minimal modification to my QMK set-up. I couldn't really use it dangling off some wires so I set out to make an extension that would slot in and be retained by one of the standoffs. I don't have a laser cutter or 3D printer so I just hacked these out of a sheet of ABS plastic following a printed template and glued them up. They're nice and solid and line up really well, despite being rather rough around some of the edges.
I'm finding the 1.5U keys a bit confusing at the moment, swapping them out for 1U keys makes it a bit easier to home my thumbs. These are certainly much easier to access than the outermost keys on the lower row or the keys below the display. I'm thinking they need to be called Lily Pads.
I got an overpriced 250×200mm sheet off Amazon, just because it was easy. It's definitely available cheaper elsewhere. I was trying to get blank black G10 / FR4 board to match the PCB but it doesn't seem easily available in small quantities (except from China). I just searched for 1.6mm sheet and this stuff came up (it's 0.06 inch but close enough). It cuts really well with a hacksaw - didn't chip or crack at all and cleans up with a file or just a craft knife. I roughed it up the surface with sandpaper to CA glue it together and they're holding together great.
The key cap set I got only had 1 and 1.5U. There are some 1.25U caps available separately online so I may well do that, thanks!