1x Arduino Uno

3x Sharp IR Distance sensors (GP2Y0A21YK0F) 

     These can be found for the lowest price on Amazon. I purchased one two-pack and one single sensor for about $30 including Prime shipping.

2x Round Clips (my design) https://www.thingiverse.com/thing:2514461

2x Mount Arms (not my design) https://www.thingiverse.com/thing:2194278

    I chose one 90cm piece and one bent 60cm arm from the list of STLs and they work just fine. The main goal of these two pieces is to give the castor sensors a farther reach from the wheelchair while still providing a strong mounting solution resistance to vibrations that may change the position of the sensor. 

2x Sensor Housing w/mount attachment (my design) https://www.thingiverse.com/thing:2514469

The three distance sensors are connected to the analog pins on the Arduino Uno board. I currently have an LCD displaying the distance measured by each of the three sensors, but it is not necessary for the final build. A piezoelectric buzzer is attached to digital pin 1 and to ground.