I bought a cheap 3v/5v AVR Programmer just to have on hand in case I ever had a firmware update issue and needed to manually reprogram a bootloader or something. These are available for a few bucks on eBay. I have yet to use it.

I believe this is the product page for the programmer: http://www.protostack.com/accessories/usbasp-avr-programmer

Update: v2 of the case makes a few improvements. Round holes for the LEDs are 1.75mm, and accept a small length of t-glase as a light pipe for the LEDs. Note that holes will print undersized due to various artifacts of FFF 3D printing and you should ream the holes with a 1.75mm drill bit (or a #50, which is what I used, and is close enough) This works nicely. Also, I added a few tabs to these sides to help hold the two case halves together, with limited success.

Also, if you got your USB ASP from China (which you likely did) make sure you're running the latest firmware or you might get errors when you use it. A great how-to on updating your USBASP firmware can be found here.

Dimensions of assembled case (not including the USB ASP connectors) is approximately 47mm x 22.9mm x 11.25mm

100% infill
0.15mm layers
0.4mm nozzle
ABS