Hi Leslie! Do you have a short sleeved black tie top? Our troupe uses those a LOT. You can tie them in the front (assuming it'll hide the bra!), or cross them over in front and tie, tuck, pin or whatever it takes to keep it closed in the back. You can add a kuchi pendant or something in the front if it looks too plain. Just a suggestion. I've used my black tie tops more than any other costume piece I have (plus I wear them over tank tops for everyday).

T-shirts make instant cholis.
When cutting the extra length away, cut so that the top is a couple inches longer than you want, plus a hem. Cut it three or even four inches longer in front than in back, so there's enough fabric left in the top to cover the bossom comfortably. Also, if you need a little more freedom of movement, open up the underarm seam at the armpit. The only thing you REALLY need to sew up is the hem, and that's because the elastic really helps out when using this technnique for a large bossom.
Choli 1 - full front, covered back. Cut the lower section off, leave enough length for a good sized hem plus a little more. You can always cut it shorter, but it's hard to make it longer! And, big chests require extra fabric for comfortable coverage. Cut the front of the neckband off, and cut the neck as low as you want it. Then turn the lower hem up, adjust the length as necessary, and run a piece of elastic thru the hem (treat the hem like a casing). The elastic helps hold things down, so to speak. You can leave the neck edge unfinished for a Gothy look, or use the cut away scraps to make a simple facing.
Choli 2 - open back. Make Choli 1, then cut away the back as desired. Can slash it horizontally or vertically or even diagonally & leave the slashed edges for a Gothy look. Or cut a deep V or U (smaller than the desired finished opening) from the shoulder (begin at the point where the neckband is sewn to the shoulder. Dont' mess with the shoulder seams) to just above your bra strap. Turn the edges under a tiny bit (like 1/4 - 1/2 ") and hem. Use the scraps to make ties to help keep everything in place across the back. You might also want to face the lowest inch or two and the point of the V.
Choli 3 - tie front. Cut the lower section off, leaving enough length for hem plus a little more. Cut the front neck away into a deep V, slightly smaller than the desired finished neck opening, and the point of the V a bit above the center of your underbra. Then cut from the V straight down through to the bottom edge. Cut two ties from the scraps, and attach one tie to one side of the top and the other tie to the other side of the top. Face or hem the neck and upper edge or the tie (or not and call it Goth), and hem the lower edge of the top & ties. You may want to add elastic in the bottom hem, of just the T-shirt part, not the ties.

Helen might jsut be a genius...Now I know what i can do with all those tops that I don't quite know what to do with!

I've done the tshirt choli out of a stretchy v nect long sleeve black T, and out of a scoop neck short sleeved baby T....

in both cases, I just left the neckline alone - cut off the bottom as the other poster described - made a dart in each side to help take in any excess - then folded the bottom up and hemmed it.

with the dart, about 1 1/2 - 2 inches deep on each side, it holds tight, so I didn't need any elastic - I'm a pretty lazy sewer, so it works out well for me!