It's strange to me that I never hear people talk about enforced proxy quality. I'm a big fan of proxy events, I have been forever, but I, too, feel the pain of the sharpied Scalding Tarn.
Sometime back in the golden ages, I was an SCG Power 9 event. My opponent cast a Null Rod off of a basic Forest, and then next turn a Tarmogoyf, and that Tarmogoyf killed me. Except the Forest was a Mox Emerald proxy done in ballpoint pen that I couldn't read, and I didn't notice it was a Mox until the turn after he illegally tapped it for mana. I called the judge but it was too late to roll back, and I lost to the Tarmogoyf, knocking me out of the event.
But there's a false dichotomy here which is annoying. There's no reason you can't run an event and say "You can run proxies but they have to be legible." For me, the best proxy is a computer printout, on a plain piece of paper, cut out and put in a sleeve in front of another card. I use these for testing and for lower-key paper events. There's never any confusion, and when effort is taken to print the image out at the right size with the right sized border, bystanders and opponents rarely notice I'm playing with proxies at all.
Is this kind of proxy possibly marked? probably. Is it as marked as a double-sleeved foil? not even close.
Sure, you're asking players to take slightly more effort when making their decks, but this seems like an incredibly low bar compared to sanctioned play. Asking them to spend a few minutes with a printer doesn't really feel in the same ballpark as asking them to spend $20,000 - which, of course, they're always welcome to do if they don't feel like making proxies.
Just asking people to make nicer proxies doesn't solve every problem everyone has with playtest cards, but it does solve a very big one, and it's really weird to me that nobody ever seems to bring it up.