Patch U+F8FF of a TrueType font with an SVG. Make sure your entire SVG is made of paths.

convert.py

from fontTools.ttLib import TTFont
from fontTools.pens.ttGlyphPen import TTGlyphPen
from fontTools.pens.cu2quPen import Cu2QuPen
from fontTools.pens.transformPen import TransformPen
from fontTools.pens.boundsPen import BoundsPen
from fontTools.misc.transform import Transform
from fontTools.svgLib import SVGPath


def patch_svg_into_font(svg_path, input_ttf, output_ttf, max_err=0.02):
    glyph_name = "uniF8FF"
    font = TTFont(input_ttf)

    # Font metrics
    upem = font["head"].unitsPerEm
    ascent = font["hhea"].ascent
    descent = font["hhea"].descent
    vbox_height = ascent - descent

    # Determine glyph advance width
    hmtx = font["hmtx"].metrics
    if glyph_name in hmtx:
        advanceWidth, _ = hmtx[glyph_name]
    else:
        first = font.getGlyphOrder()[0]
        advanceWidth = hmtx.get(first, (upem, 0))[0]

    tt_pen = TTGlyphPen(None)
    cu2qu = Cu2QuPen(tt_pen, max_err)
    svg = SVGPath(svg_path)
    svg.draw(cu2qu)
    glyph_raw = tt_pen.glyph()

    bpen = BoundsPen(None)
    glyph_raw.draw(bpen, None)
    xMin, yMin, xMax, yMax = bpen.bounds
    raw_w = xMax - xMin
    raw_h = yMax - yMin

    sx = advanceWidth / raw_w
    sy = vbox_height / raw_h
    scale = min(sx, sy)

    target_cx = advanceWidth / 2
    target_cy = (ascent + descent) / 2

    glyph_cx = (xMin + xMax) / 2
    glyph_cy = (yMin + yMax) / 2

    dx = target_cx - scale * glyph_cx
    dy = target_cy + scale * glyph_cy

    tt_pen2 = TTGlyphPen(None)
    cu2qu2 = Cu2QuPen(tt_pen2, max_err)
    transform = Transform(scale, 0, 0, -scale, dx, dy)
    tpen = TransformPen(cu2qu2, transform)

    svg = SVGPath(svg_path)
    svg.draw(tpen)
    glyph = tt_pen2.glyph()

    order = font.getGlyphOrder()
    if glyph_name not in order:
        order = order + [glyph_name]
        font.setGlyphOrder(order)
    font["glyf"][glyph_name] = glyph

    lsb = int(round((advanceWidth - raw_w * scale) / 2))
    font["hmtx"].metrics[glyph_name] = (int(round(advanceWidth)), lsb)

    font.save(output_ttf)
    print(f"Saved {output_ttf}")