My solution has been to use a chess font like Chess Merida or Chess Cases.
With such a font for example the start position is written like this:
1222222223
4tMvWlVmT5
4OoOoOoOo5
4 + + + +5
4+ + + + 5
4 + + + +5
4+ + + + 5
4pPpPpPpP5
4RnBqKbNr5
7888888889
And (assuming the line-height is set to the height of the font) it looks like this (Here using Chess Merida as font):

So i wrote this Python script which converts from fen to this format. Call this script (assuming you named it fen2diag.py) with python fen2diag.py "<the fen>"
and it prints out the diagram string.
import sys
def fen2diag(fen, borders=False):
"""
Convert a fen to a diagram string used by fonts like
'Chess Merida' and 'Chess Cases'.
fen: The fen. For example the fen for the startposition is
'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1'.
borders: If the returned diagram string shall have borders.
Returns the diagram string.
"""
# We dont need anything except the piece positions.
fen = fen[:fen.find(' ')]
# Transposition table for the black pieces.
# White pieces are the same in both formats.
t = {'k': 'l', 'q': 'w', 'r': 't', 'b': 'v', 'n': 'm', 'p': 'o'}
# If the current square is a white square or not.
w = False
def todiagletter(fenletter):
""""
Return the diagram letter corresponding to the letter in the fen.
"""
nonlocal borders, w
w = not w
if fenletter == '/':
# In the diagram font these are the characters for the diagram borders:
# '1' upper left, '2' upper, '3' upper right,
# '4' left, '5' right,
# '7' bottom left, '8' bottom, '9' bottom right
return '5\n4' if borders else '\n'
else:
# this code handles numbers in the fen, denoting empty squares.
try:
# this is a number between 1 and 8.
n = int(fenletter)
# This will be a string denoting empty squares.
# Would be eg. ' + + + +' for an empty eight rank.
spaces = []
while n > 0:
# In the diagram font ' ' denotes a white square
# and '+' denotes a black square.
spaces.append(' ' if w else '+')
w = not w
n -= 1
w = not w
return ''.join(spaces)
# this code handles piece letters in the fen.
except ValueError:
# a black piece
if fenletter in t:
fenletter = t[fenletter]
# In the diagram font lowercase letters denote
# pieces on white squares and uppercase letters
# denote pieces on black squares.
return fenletter.lower() if w else fenletter.upper()
diagram = ''.join(map(todiagletter, fen))
if borders:
return f'1222222223\n4{diagram}5\n7888888889'
else:
return diagram
if __name__ == '__main__':
print(fen2diag(sys.argv[1], borders=True))
These Diagram fonts also support squares marked with dots or stars, another type of border, round border corners, numbers/letters on the left/bottom borders denoting the rows/columns. I have not included this in the script. Feel free to update my code.
Chessbase has also made a font family (starting with 'DiagramTT...') which supports even more stuff (like pieces turned by 180°) but this font maps things to different code points, also for black squares two letters are taken, one for the background and one for the piece.