>>1783307option to consider is using a stem
If the bike has a quill stem you can clamp a threadless stem on it (with a lot of shimming), and if it has a threadless stem with a large enough stack, you can replace spacers with another threadless stem.
Threadless stem is then rigid and will take 4 bolts at the other end to attach a sandwich plate (just 2 plates of metal with corresponding holes drilled, to go on either side of the basket, and clamp it with bolts). Use washers too. Wa la. Much more secure than any plastic bracket.
