I am wondering if there is an indicator that I can use to close a trade a certain number of bars after opening.
Unfortunately, this simple logic cannot be implemented easily in FSB and FST. The reason for that is that the indicators can use information only from the market data - time, price and volume. An indicator doesn't have access to account data including positions and orders. Therefore, there is no way for an indicator to know when or at what price a position is open.
However, this idea is not completely impossible for implementation for FST. Someone can add this option in the expert advisor - MT4-FST Expert.mql. A function written in the expert have full access to all info, including currently open position and the time - when they was opened. So that will not be difficult for a programmer to write a short script that checks the current bar or tick time and to close it, if it is necessary.