Make your own Concrete Letters!
The mold walls are designed with a two degree skewness.
So you should be able to get the hardened concrete out easily if you fill the mold just 3/4 with cement.
Oil all shells prior filling in the Cement....
The shown Sample "R", half dried, still to be finished by possible sanding and painting...
In work now the (R)ELAX :-)
See also the SMILY Latern:
Set your Slicer to 100% Infill and one shell perimeter.
Preferable to be printed with PETG.