Update 12/28/2016: Everything is printed and fits! I ended up going with a different fan because I was worried about the CFM not being sufficient. I updated the link below. I have not built my case yet, so I have not been able to test the efficiency of this filter, but I assume it to be plenty to serve the task at hand. Make sure you download the new files I uploaded, I made a few tweaks before printing to make sure everything lined up.
I designed a bracket and enclosure for attaching a 120mm fan, activated carbon filter, and HEPA filter to a 3D printer case for filtering ABS fumes. I have been looking for a model that includes parts that can be sourced from Amazon and Home Depot, but have yet to come across one.
The following parts are what I am using:
Carbon Filter: https://www.amazon.com/gp/product/B000U204W2/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1
HEPA Filter: https://www.amazon.com/gp/product/B0007INDN6/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1
All the parts are fairly inexpensive. You can certainly source any 120mm PC fan, I only chose this one for simplicity of hooking up to a USB port for power. The "Inside Case Fan Bracket" would go inside of your acrylic/Plexiglas case. You would then cut out the 120mm fan hole in the acrylic and drill for screws. I am using a router to do this part. The fan would sit right outside the case, attached to the fan bracket with #8-32 x 3/4" nut and bolt. The carbon filter would be cut to 96mm W x 60mm H and placed in outside section of the fan duct. The HEPA filter is the same size, and would be placed in the HEPA filter bracket, with the gasket facing out. The fan duct and HEPA bracket would be fastened with #8-32 x 1/2" nut and bolt. This should provide a snug fit, that will apply some pressure to the gasket of the HEPA filter, hopefully preventing any leakage. (Update 12/28/2016: I used the long black screws and nuts that came with the fan to attach the fan, bracket, and duct to the plexiglass. This means you only need 4 screws to attach the filter bracket. You could probably find something on here to print and use as a nut and bolt.)
The filter bracket allows you to easily replace the HEPA and carbon filters, without removing the fan or detaching from the case. The inside case fan bracket can also be used as a template for cutting the acrylic/plexiglass.
In total, the HEPA filter, carbon filter, fan and screws all cost about $30.
You can go with lower infill, I only did 100% because I didn't want to waste a day of printing for it to fail. The supports are required for all but the inside case fan bracket.