How can I make a dog scratching thing to keep his nails filed?

If you want to file his nails, you can get an electric nail grinder for dogs. I'd think he'd have to a be a patient, quiet dog for that though. Otherwise you can get dog nail clippers. There are two types. One for little nails and one for large dogs. Easy peasy once he gets used to having his nails clipped. Otherwise take him to a groomer, they will do it.

Make an appointment with the vet you use to clip his nails and to instruct you on how to do it. You MUST be aware of the quik which you do not want to cut into. I personally take my guy to the groomer who will clip his nails for $5. I am a customer of hers, so I don't know if the price will be accurate for you or your area.

Either learn how to trim and file his nails, or take him to a groomer regularly... they usually only charge a few bucks for it. I had a client whose red zone pit bull would go nuts when you pulled out the nail clippers. I would get her to sit next to me on the couch, and then lean on her with all my weight and wrangle a paw out, and nip away. Be sure to have Quick Stop in case you nip a vein, just take a bit of it and press into the end of the nail for a minute. This dog would tolerate nail trimmings after I had been doing it for awhile, but she liked the electric nail file I got her... I used the large one at the bottom.... https://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=dog+electric+nail+file

We use a manual clipper and a dremmel.

Some Petsmarts do clipping Im not sure of the cost.

Regular walks on concrete or asphalt.

Walk him on concrete... and there pet nail grinders made by companies like Dremel and Oster... that have a rotary sanding drum... they turn at a much lower speed so the nails don't get hot...PediPaws is another one

My vet always recommended taking my dog for walks on the sidewalk or asphalt, it'll naturally do it. Here is an article that might help - http://www.dogtrainingnation.com/how-to-train-a-dog/building-a-dog-nail-scratchboard/

Dogs don't naturally scratch at things to groom their claws. The wear on their nails comes from the surfaces they walk on, so Bijous has a great suggestion that helps both dog and owner! It doesn't cost much to take your dog to the groomer or vet for a nail trim, unless you want to do it yourself. But BE CAREFUL! It's very easy to trim a dog's nails too far causing bleeding that's difficult to stop and pain. Usually best to let those who know how do it.