I think I need to install a skimmer due to serious trees and I want to
make it myself. If I can't - I'll just deal with a mucky pond at this
point (getting fed up with the process).

The concept seems simple enough. A container, a spillway cut into the
container, a pump in the bottom, netting or crate to catch the junk.

BUT....

1. What about the swinging door thing? Do I need that? I think so
because it will still work with raised or lowered pond levels. Right?
Is that correct thinking on the door issue? If that's true, I guess
one would want to get a replacement part door thingy from a premade
skimmer manufacturer? Or - how would one create a proper door? Yet I
see some that have no flappy door?

2. Also, I'm interested in a smallish footprint for this thing. I
have an 1800 gph pondmaster mag pump. Is there any rule about
vault/container sizes per the power of the pump? Or is it fair game
for me in choosing the size of the container? I read here someone
used a plastic file cabinet box. It would be great if I could use
something similarly smallish.

3. What happens if the water level drops below the spillway? My pump
would die. How do you prevent this? Does the flappy door idea
prevent this from happening cuz you can sink the lowest level of the
opening lower?

Any pictures?