For good performance, 17 cm of straight wire will work as a 433 MHz antenna. Shorter antennas lead to lower operating range.
The module pictured has a built in antenna, which was designed to work in a non-conductive housing. For best performance (and possibly, for the thing to work at all) you will need to cut the connection to that antenna, as it will short-circuit the transmitter output when enclosed in a metal box.
It may work to cut at the beginning of the matching "coil" and connect the wire antenna to the via to the right of the arrow tip.
