HEPA Filters are made for clean rooms and easily clog up in a home environment. After reading many studies done on filters, we've concluded that getting a furnace filter is the best choice. Today there are plenty of sub-micron grade furnace filters, and luckilly for us, they are all priced based on micron size (so get the biggest one you can with the highest filtration rating possible.)
Also doing this seems to create a better air path (blocks off any cracks that air can bypass the filters with, as they are usually designed to do - as HEPA plugs up and that'd wear the motor out.
Step 1: Materials List
-A "flat" filter based air cleaner, technically any of them work that I've tried (all of them were holmes brand but others seem like they'd work)
-Allergen grade furnace filter (all of them are priced by how small of particles they filter, not by size. GET THE BIGGEST ONE of the rated filtration you desire to use)
-Gloves (to keep your hands from getting cut on the mesh)
Step 2: Make a Template for Future Use
Take your old filter out of the machine, and put it on the cardboard. Trace, and cut out. You can make more for each of your filters to make sure you get enough for everything, or just use one.
Cut this out when you're done.
Step 3: Known Filter Sizes
Holmes G Filter (HAPF21)
Holmes E Filter (AOR115, AOR118, HAP115)
Holmes "625" (replaces the mutliple filters in the unit with one) (HAP625)
If anyone knows more, feel free to comment and I'll add them.
Step 4: Layout and Cutout
Mark with a marker and cut out. Be careful about the sharp electrostatic mesh on the back, and try to not crush the folds as this would hamper the surface area per square inch of the filter.
It helps to cut down the fold of filter, and to mark or make note of what side goes down (esp for the filtrete 1250's as they are similar on both sides)
Step 5: Install the Filter!
Tape around the edges of the filter with duct tape, and push the cover back on the filter unit. Note that it may become harder to remove the top...
NOTE (The pictured is a 1250 that has mesh on both sides, but the wording on the side of the filter is readable - right side up)