A Substitute Dust Filter
On the back of many Tektronix scopes such as the 7000 series, there's a captive dust filter over the fan. The original filters are an open-cell polyurethane foam, that does not age well. Once this has started crumbling to to sticky crud it's unwise to leave it in place. The fragments are both sticky and corrosive, and can get blown through the insides of the scope. Not to mention that dust now enters too.
A simple, cheap and very functional substitute is the humble kitchen fiber scouring pads. These are about 7mm thick, and easily cut with scissors to fit the filter mounting. They are rigid, and open enough to allow unrestricted airflow, while close enough to catch dust. There are often jumbo size ones available too, if the standard size are not big enough for some filter mounts.