This is the schematic I've used, based on a MC14050B Non-inverting Hex-buffer to adapt SD card 3.3V levels to 5V levels of Arduino.
As 3.3V supply I've used this one.
Here's the realization on a stripboard:
The SD card holder has been soldered on a small piece of stipboard (on the strips side):
Here it is:
Here's another implementation following the same schematics. It uses a more stripboard-friendly version of an SD card adapter salvaged from the first version of the official Arduino Ethernet Shield (that has this adapter on-board but does not have all the circuitry to correctly interface to an SD card).