You can use a fixed width font by specifying -builtin fixed. You can also specify your own font with the -font flag. The fontfile is either a BDF file from the X window system or a PBM file. If the fontfile is a PBM file, it is created in a very specific way.
M Do a screen grab or window dump of that text, using for instance xwd , xgrabsc , or screendump .
Convert the result into a pbm file. If necessary, use pnmcut to remove every- thing except the text. Finally, run it through pnmcrop to make sure the edges are right up against the text.