waterbug_guy used to be here a lot. He liked to play around with ideas himself. He also experimented and explained his reasoning. He has a link to his diy skimmer
HERE.

Thanks. How about sizing of skimmer box and sizing of pump to work with the skimmer?

bump, I really need some help, please experts??

I think I noticed that waterbug_guy added to a post recently, but I hadn't seen him for months. I don't really know anything about big ponds like yours and I don't know how much flow is needed to draw from such a large area.
However, except for possible extra flow needs, you might not need an extra pump. It depends on the layout of the inlet on your existing pump. If it can be connected to a hose, then you just connect a skimmer with a long enough hose to get well away from the fountain. This wouldn't work so well if the fountain is in the middle, but that would cause a general problem anyway since the fountain would drive water and needles away in all directions and the flow could overpower any current of the skimmer.

The skimmer box can be any size you like, can even be a pond (skimmer between 2 ponds). The width of the weir door (flap) determines the pump size...kind of. Really pretty much any size pump will work with any reasonable width weir.

Technically you could compute the amount of water desired over the weir. So say 12" wide weir at 1/8" deep is about 1.4 sq in of water which is about the size of a 1.5" pipe. Looking at a flow table for gravity to low flow (6'/sec) you'd need a 2100 GPH. With 1/16" over the weir a 1050 GPH pump, for 1/32" a 525 GPH pump.

Lot's and lots of pond owners, including high end pond owners, believe, and will always believe, that the bigger the pump the more "powerful" the skimmer. I've also seen many posts from "experts" saying a waterfall is needed to push debris to the skimmer. That all ignores the physics of why a skimmer works which is based on surface tension. The thinner the water going over the weir the stronger the surface tension pulling debris into the skimmer. So the smaller the pump the greater the pull distance. That assumes a level weir and no leakage around the edges which isn't realistic.

People wanting to spend a lot of money on a pond will pair a 4000 GPH pump with a 10" weir and it will indeed pull stuff into the skimmer so they assume a huge pump is needed, or is better. It isn't. But it's impressive to people impressed by excess which this hobby can certainly be about.

Bottom line if you use a 500 GPH pump or bigger and a 10-12" weir you should be OK. And that single skimmer will clear the surface of even a 25x35' pond. Stuff may get hung up on sides, but no skimmer can deal with that. And on windy days stuff may blow away from the skimmer which again a bigger pump isn't going to solve.

Really skimmer design generally has much more to do with the owner and how knowledgeable and/or if their goal is to have lots of equipment.

If you make a skimmer the main thing to get right are getting the walls and floor where the weir is installed to be square and walls parallel so as the weir moves up and down there a minimal gap between the weir and wall. Messing that up then requires a larger pump to overcome the loss around the edges. The corrugated plastic makes getting the weir door perfect pretty each since it's so easy to cut. Trim the edges that move against the wall at 45 degrees instead of 90 degrees so the final trim is just a single layer of thin plastic.