after few months of owning older Roomba 581 I came to conclusion that the microfiber air filter just sucks compared to my parent's roomba 700series with HEPA filters. Fine dust flows throught the filter into the air and also the filter gets clogged really easily reducing the airflow to minimum.
I searched for some upgrade. I found some HEPA filters on amazon
for Roomba 500 series but according to reviews it was not so great. Also shipping to my contry would be a problem.
So I decided to build my own solution - I bought HEPA air filter for Roomba 800 on ebay
for around $2 and pack of stock air filters for my Roomba 581 for around $3.
Step zero - I took stock filter apart and removed (brute force) the black grid holding microfiber filter (forget to take a photo)
First I removed the black plastics around the filter itself.
(it is dirty because I duct-taped it to roomba exhaust to see how much dust is coming to the air while vacuuming with stock filter...quite a lot)
Then I cut the filter to match stock filter dimensions
Along the edges I poured some glue to seal it.
Larger areas were sealed using silicone (white parts):
Fits really nice into roomba's dirt bin.
Now my roomba has much better airflow even with quite a lot of fine dust trapped in the filter. Also realeases less dust in the air when vacuuming.
First I was worried that the HEPA is too thick and roomba will have problems pushing air through - but it was the opposite - this folded HEPA filter has higher surface area and makes even less resistance to airflow compared to "see through" stock one.
Possible improvement is to remove fine plastics mesh covering the actual HEPA filter - when fine dirt gets between the HEPA and this mesh, it is hard to clean.It will be less protected but I think it is not so big issue.