They don't necessarily fit to the fender so you don't need to worry about holes. You can pick them up pretty cheaply (at least here in the UK) so I wouldn't bother making one.

This one is £6.99 which is about USD10, but the good thing about it is that if you wanted to make your own, you can see how you would go about doing it.

https://www.bankruptbikeparts.co.uk/index.php?route=product/product&product_id=21396

I'd make templates in cardboard, see if they fitted and then transfer to either sheet metal (a bit of a pain) or something like polypropylene sheet. If you were being super frugal you could just use a waterproof card, like the stuff you see on estate agent signs. The material I chose would be what I could find.

There are different ways to clip it on, if you use something like a jubilee clip, I'd line it with some old inner tube to stop it scratching your bike.