As i don't own some sort of cutter i just went with 3D Printing. I will probably add the Letters to Thingieverse so you can do that (PITA) to yourself :-D

Since i wanted to have the Clock like half a meter wide each Letterbox has a outer dimension of 40x40x20mm. Nine pieces took about 1.5h to print (Yepp ~ 20h of Printing, but not that hard).

In Retrospect i would not recommend to print each Box seperately. Maybe you can combine some Boxes at the Slicer so it would be easier to combine.

If your printer is Big enough you can also just print all at once ^^.

I did it the labourious way - glue each box to the next one. One by One, Row by Row.

As you can See the Backside has some issues ... blobs of XTC 3D and some pieces of the underlaying Material. Nothing to bad - since not visible.

After that i added some layers of Black Paint and XTC 3D to the Clockface. Originally i planned to use it as visible Part. But the Finish of the other side did not meet my expectations.

Maybe some more Painting and sanding? But the solution is also really enjoyable. But Later more.

Beside the Letterboxes i also added the Holders to the Edges which will be screwn down to the Plywood.
With the Additional Glue and XTC 3D and Paint it is really sturdy.