I would consider the LED type. I note these normally need 39mm behind them for the out door type. I assume you are using extra low voltage not low voltage?
Low voltage = 50vac - 1000vac
Reduced Low voltage = 55 - 0 - 55 or 64 - 0 - 64 if three phase
Extra low voltage = Under 50 volt ac
DC voltages are different.
Most 12vac lights are classed as SELV or separated extra low voltage.
If you are sticking in your own LED as standard the white ones are around 3.5 volt and have a max reverse volts of around 5 volt so you will also need diodes and resistors to limit current and stop reverse voltage.
So for example a 30ma 3.2v led will need a diode and a 270 ohm resistor if supplied from 12vac. At 5mm you should be able to use 10 which would cost around £20 including resistors and diodes at Maplins prices. Nice little project.
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