For Canon full-frame DSLRs, the widest commonly available rectilinear option mentioned here is the Canon EF 11-24mm f/4L USM, which is wider than a 12-24mm while still aiming to keep straight lines straight.

If you need an even wider field of view than that, the practical options are:

• use a fisheye lens, if curved distortion is acceptable
• stitch multiple overlapping frames into a panorama

Attachments generally won’t help much here. Extremely wide full-frame lenses typically don’t accept normal front-mounted accessories because of their large bulbous front element; some only allow rear filters, and those do not make the lens wider.

Also keep in mind that going wider than 12mm on full frame is difficult to use well: flare becomes more likely, avoiding bright light sources is harder, and distortion can be more noticeable. For many situations, stitching images can produce a wider result with better detail and potentially less distortion than using an ultra-ultra-wide lens.