I'm with Bearygood about the fitting. It's done without a bra in order to get accurate measurements.

I have sports bras but find they're still firm around my rib cage. Infact, I think they'd have to be in order to support you while exercising/playing sport.

I've never worn them or spoken to anyone who's worn them, but do those gel type stick on cups work? Would they work for one night at this function? Just an idea.

If you were having a dress made, or buying one especially for the graduation, how about a built in bra?

I know that doesn't help your every day need though, 'cos I'm another who
doesn't a wear bra unless I have to leave the house.