You will need a coin bra. You can buy ready made or make your own to save money. If you are using a bra as a base make sure to change the straps so that it does look like a piece of costume rather than a piece of lingerie
Our cholis have open backs and there is nothing worse than seeing lingerie straps criss crossing what should be a beautiful bare back. So a halter neck works well but can be a little uncomfortable - adding a small length of elastic to the strap eases things so there is not so much pressure. Or you can move the shoulder straps so they sit over the choli but you may need to pin them so they don't fall off your shoulders. Another trick is to thread a thin cord or shoelace under the bra straps and tie it so it sits along the same line as the choli neck tie.
Coins are heavy so you’ll need a strong base to withstand the weight and its best to secure each coin before adding the next so that should a thread break you only lose one coin.
You can personalise your bra with little charms, bits of jewellery, dangle chains and of course loads of coins but go easy on sparkle and cowrie shells. There are plenty of online sources for coins, pressed metal stamps, bits of dangle chain and kuchi pendants.
If you're worried about the weight then using layers of dangle chain is much lighter than a full coin bra and can be more flattering on a bigger bosom