Where would I get those little things where there's a grommet in the canvas and you put it over the thingamajiggie and twist it to hold it on? I have a bunch of old heavy military canvas and I'm thinking of making canvas curtains for the boat.
Those are pretty good, and utilize a standard grommet which is nice. Really depends what you are covering. Like Jjammer said twist locks(or similar like you linked) on the corners take the load, and snaps in between secure the cover. This would be for larger covers... if just smaller dash, console, companionway cover then snaps likely fine and if done right can be very secure. The key to snaps is lubricating them once a year with a dab of T-9 to avoid corrosion that make them get stuck/break/rip out.
I was a sailmaker for many years and ran a Service loft so have seen/ repaired a lot of set ups... that being said every cover is different so may be worth consulting a local sailmaker/ canvas shop if you can.
Just to note, on our cockpit cover we use the quarter turns AND 1/4" elastic shock cord. The shock cord is just tied in a single overhand knot, pushed through a brass grommet (from inside) in the cover. The shock cord loops go over the cleats and at the forward corners hook onto the jib track. The shock cord loops last at least 5 years and I just replaced some this week, with the rest in good shape still. Sailrite has shock cord hook fittings too, for areas where you may not have a cleat. This cover has had zero issues in storms, etc over the past 6 seasons (24/7/365) and that is with the ends raised up for good ventilation. I'll get and post a picture if anyone wants more details. That same shock cord can be sewn into the hem of an outboard motor cover too.
This shows the elastic snapped over a cleat, and also the hook that is below it, but was not really needed. In some areas, like the port and starboard jib winches, we have a hook on either side. I note that the loops stay in the canvas and do not come out as they stay in a bowed shape.