The hydraulics in both of my drivable Sterlings have been more than adequate for keeping the top tightly closed. My cars have a few mystery clunks over bumps, but nothing objectionable...and nothing that I think would be remedied by any mechanical latches. The problem is that, although the canopy is quite rigid, it is also quite large. Securing it only on the sides might camp it down bit, but it still might teeter-totter enough of a fraction of an inch to still have a little play/a little "loose" sound. A builder could create a three-point latch mechanism, but with any of the above, you're adding a whole lot of complexity to solve a problem that (thankfully) really isn't a significant problem (and you'd be greatly increasing the chance of a failure that gets you stuck INSIDE of the car...which, in practical terms, is more of a real concern than holding the top down.)
In my opinion, the only thing a builder needs to hold down the canopy is just a properly chosen hydraulic system (or carefully chosen linear actuators) that have a check valve to hold the load in the "up" position and also a check valve to hold it in the "down" position. Not all pumps have both of these options. It is a commonly overlooked nuance to designing a hydraulic system that works very well for our cars.
That said, I'm pretty sure that one of my cars does NOT have a check valve in the down position, and the top stays down pretty tightly for pretty long. If I'm ever on a longer trip and the top starts sounding loose, I just bump the switch for half a second and it tightens it again. Holding the top down really isn't a problem.
I think the biggest mistakes people make with the hydraulics are 1) sloppy or old wiring that isn't reliable and 2) sloppy, old, or inadequate plumbing that is always leaking. If someone sets up a top using a good pump, hearty hydraulic lines (bought from a custom shop like AeroQuip, etc), a clean-and-robust circuit to control it all, and carefully refined weatherstripping, they will want for nothing. The top will work reliably, will stay up or down when you want it to, and will be quiet and watertight with no further modification. Mechanical clasps wouldn't be 'wrong,' but they really aren't needed. Better to save all of those tricks and creativity for finding good ways to latch the hood. (I've heard of about a half-dozen Sterlings loosing their hoods while driving...sometimes breaking that very expensive windshield.)
Tom,
I had a Sterling that had hydraulics AND those old hold-down clasps, and, after a while, I stopped fastening the clasps. The canopy had the same little bit of play with or without them. I definitely never had the impression that things were sloppily shaking around. Just a little bit of a noise over rough roads, which I think I could have virtually eliminated with better weather-stripping. That car had weather-stripping that was dry rotted pretty much everywhere.
