Use a regular ladies tee shirt. I see no reason to need to alter one as ladies fitted tees are readily available plenty cheap at places like Walmart, I have tons.
As to the figure issues: learn from Stacy & Clinton
(TLC's What Not to Wear- GREAT advice)
- wear the right undergarments, in this case a slimmer of some sort would probably work nicely.
- wear clothes that FIT, not squeeze. If you don't know how clothes should look on your body when they fit properly... well you are about in with 63% of Americans, most people have no clue what proper fit is. It's not something I can explain via text, I'd need to see you in clothes to explain what's right & what needs adjusting. I can do that via PM if you'd like.
Especially if you have more curve than the character you are playing, wearing something that looks good on you
will read much better than something that fits like it was painted on (like the character being played) which results in making you look terrible. It's a fine balance.