This is a parametric caster modeled in the form of a MakerBot. It is derived from the OpenSCad script Printable Key Rings by catarina http://www.thingiverse.com/thing:1378
The script allows easy adjustments of the caster's height, width, wheel width, wheel diameter, wheel position, bolt hole size, counter bore holes etc. Echo statements within the script provide approximate mount height and required bolt lengths for the wheel base. On the wheels, is an embossed profile of hoeken's Gator clip http://www.thingiverse.com/thing:507.
OpenSCad is still new to me. I encountered difficulty in getting the embossed profiles from the extruded dxf to translate correctly with adjustments to the wheel base parameter. If someone can enlighten me on solving this issue, it will be much appreciated.
I have uploaded the .zip file of the script along with catarina's included code and hoeken's dxf. Also included are the .stl files and g-code for my print. This has a wheel diameter of 35mm, wheel width of 13mm and a mount height of approximately 45mm. The wheelblock takes approximately 69 mins to print while the wheel takes 23 mins.

I recommend using Netfabb Studio (http://www.netfabb.com) to manage the .stl files generated from the script. This software (Basic Version) is free to download. I used it to easily repair and properly orient the stl before sending to Skeinforge.
Once the parts are fabricated, assemble as shown in the photos. It may not be necessary to install two wheels, one may be suitable ( I am yet to test this). Utilize washers to offset the wheels from the wheel block and the wheel block from the platform. This would reduce the contact area and aid movement of the parts. When tightening the bolts, ensure that the wheels spin freely and the wheel block can easily pivot 360 degrees under the platform. I used hot glue to glue rubber strips along the perimeter of the wheels to aid traction.