Where to advertise a postdoctoral position to widen the diversity of applicants?

https://jobrxiv.org/ and twitter although I don't know how much math people use either. For my bio-related field they're pretty good.

You'll probably get plenty of diversity in your applicant pool but you'll end up removing most of it when you filter for prestige PhD institutions and advisors. You should try to remember that URMs from second tier PhD institutions are just as smart in aggregate as the population from Stanford or whatever, but are likely to publish worse and have fewer opportunities. Evaluate them based on who's outperformed for their grad environment and in general you'll probably have a bunch of applicants you wouldn't have otherwise considered. (This is also useful for identifying the halfwits from Harvard who have underperformed despite every possible advantage - don't hire them).

The typical advise is that men tend to apply even if they are not a perfect fit for the position, while women tend to only apply if they feel they satisfy all the conditions. Thus, do not exaggerate the requirements in the job posting.

If your university/department/faculty website has lots of pictures of smiling students, make sure the pictures show diversity. According to some research (not public), this is not noticed by the majority but elicits positive attention from some minorities.

An illustrative link of how typical the typical advice is: https://hbr.org/2014/08/why-women-dont-apply-for-jobs-unless-theyre-100-qualified

Avoid tight deadlines

I am not in your field and not even in the same country, but one piece of advice I would give is to ensure that there is a reasonable span of time between the advertisement and the deadline. It can often take some time for advertisements to actually get noticed, because most people do not check a job site every day, and many people will have very large volumes of electronic mail (hence, it may take a while for them to actually see an advert posted to a mailing-list). Given that you mention a desire to recruit from a diverse pool, it is worth keeping in mind that people with poor health and/or caring responsibilities may find it harder to check for job advertisements regularly and assemble an application in a timely manner.

Basically, you want to avoid situations such as:

I would love to apply for this, but the deadline is 5pm TODAY. If only I had seen the advert last week, I might have been able to apply.

Oh, this job looks fascinating, but it will take a lot of effort to reformat my CV to show me in the best light. Oh, and the deadline is 9am on Monday. More weekend working. Oh, and I had better contact Prof. Domingo to ask for a reference. He never answers electronic mail on the weekend, so I will have to 'phone him and hope he will not mind.

Ah, this would be a dream job. But the deadline is less than 48 hours away, and I promised to take the kids to the beach tomorrow. Would I be a bad mother if I reneged on my promise? I cannot even blame the weather forecast this time! Or maybe, I can take the laptop with me, and be very careful not to get any sand or seawater in it.

Why do I always have to get ill at the wrong time? The mailing-list had an advertisement for this great job a week ago, but by the time I got out of hospital and went through my mailbox properly, I had missed the deadline.