I have a 3/4 inch PVC line that is draining from my water heater pan through an exterior wall. I want to add some bug screen to it so that no roaches or other unwanted pests crawl up it. From what I have seen online, I have to buy a massive amount of bug screen and figure out how to attach it. Is there any other way to insect proof a PVC pipe besides having to buy so much bug screen? If not, what is the best way to attach bug screen to an exterior PVC pipe?
Bunch the screen over the end of the pipe bent back along the nearby sides of the pipe, Then secure in place with a suitably sized screw gear clamp.
Something similar to @michaelkaras answer is a spark arrester over the outlet.
These are used in fire-risk areas on vehicle exhausts to stop hot sparks or embers. A flat layer would reduce air flow, so fashioning a "balloon" shape. or a tube with a closed end is common and effective.
A better solution may be to install a trap like this one: https://www.homedepot.com/p/Charlotte-Pipe-3-4-in-PVC-Sch-40-Condensate-Running-Bend-Trap-PVC027010800HD/204837401 ( or a P-trap).
I would be concerned that any type of screening will become clogged with sediment and impurities in the drain--which could leave you with damage due to water backing up.