Go for another bracket...
If it is welded, whoever does it will need to source parts, invest in disc tab jig, strip paint, prep frame, prep tab, prep reinforcement, weld, clean up, invoice. And that's without the (possible) need to re-heat treat the frame (solution heat, quench, realign, age...).
It's a PITA, but anyone competent enough (and with enough liability insurance) to weld something safety critical, such as a disc tab, to a Cannondale not designed for one is going to charge a whole lot more than £30 for that.
All the best,
Pause, take a breath and ask yourself: "What would Rastamouse