Niche targeting so difficult

Where I work, we're an advertising agency - and we often do our best to target niche groups with relevant and attractive messages for our clients. It's advertising - we're not talking about long-term engagement here - so we need to be targeted, punchy and we need candidates to quickly comprehend the deal.

One of the campaigns I'm working on at the moment requires people who could *quite literally* come from any career background. It's really not about what they've done; it's more about the type of person they are and the attributes they possess.

What we've discovered, is there are certain talent pools that these people appear in, and - without giving the game away - there might be some in Law, some in Advertising, and some in - say - Warehousing.

Anyway, my point is that once we've nailed down these groups, we need to uncover the media these people hang-out in (note: not looking for jobseeking media, we go for the passive market first). So once we've found an array of websites or communities for individuals working in Law, Advertising and Warehousing, we end up with maybe 20 different sites, all of which have marginally different specs for their banner advertising.

Back to the point in hand. Right now, my team is creating 22 different banners in order for us to - well - do our job properly. Isn't this unfair on the client who has to pay for all this development, not to mention the time it takes to coordinate and despatch all this stuff?

I'm calling for universal banner sizes - you can't build a site that advertisers will use without sticking to them. What say you?