Lots of folks use FontForge to create fonts. Very complex, I think.
I use a freeware editor called TypeLight from
http://www.cr8software.net/typelight.html. They also offer a demo version of the paid Pro version that is fully functional except it only saves 50 characters. That should be plenty for doing a few symbols and designs.
With the Pro version Type 3.2 (
http://www.cr8software.net/type.html), you can import an SVG image. I use free Inkscape to do a bitmap tracing of an existing image and make an SVG image from that, or create an SVG image right in Inkscape. You can Import that SVG image into Type 3.2 to create a new character glyph in your small custom font. Which you then embed in your epub.