I reckon this idea. OP, you should offer a free amount of BTC to deposit so that testers will come you and just give a certain number of traders so that it won't be too heavy for your budget.

And once these testers are done conducting the test trade on your exchange, you can give them a small reward like 0.0005BTC - 0.001BTC. It depends on how much budget you have but that small reward would suffice IMO.