In micing a choir, it might seem important to consider the 3:1 rule. When multiple mics are mixed to the same channel, the distance between mics should be at least three times the mic-to-source distance.
This prevents phase interference between mics (comb filtering), which is a series of peaks and dips in the frequency response - a colored, hollow sound.
The 3:1 rule cannot be applied to micing a choir with a few mics. Why? Most of the singers are somewhere between the mics, and those singers will be picked up with some phase interference.
However, since each singer is in a different position relative to the mics, each singer is heard with a different coloration. The effect averages out over all the singers and so is not very audible.
Once the mics are placed, you need a way to hold them in position. Mic cables can lose their orientation as the mic cable uncoils over time, or the mics can swing back and forth in a breeze.
Some choir mics have a built-in hanger which comes with a tiny crossbar or pipe.
You thread a fish line through this pipe and attach the line to screw hooks in the side walls (See the example photo of a miniature choir mic above to view how this should look.). The guy wire keeps the mics oriented toward the choir.
If hanging mics is not an option, you might try making some clear plastic mic stands of Lexan corner molding. The stands can be cut to the desired height and mounted to or near the choir rail.
What if the choir is under a balcony? Try mounting some supercardioid boundary mics to the bottom surface of the balcony, near its front edge.