The Simpson things are called strong ties. There is a whole line of them, designed to make Roofing framing much easier, and to bring structures, particularly light frame structures or "stick-built" structures up to the various hurricane codes. A bit pricey, but I love them too.
Due to the price, many people use Clear polycrylic panels rather than plexiglass or gas the much more expensive Lexan. As it turns out, the more opaque the plastic is, the greater its UV resistance, and thus the longer it will last. Plexiglass and the truly clear but cheap polycarbonate panels are prone to glazing and cracking over time, particularly at penetrations for screws and the like, before finally becoming brittle and falling away after some number of years. You should also remember that they expand thermally far more than the wood to which they are attached. And contract as well. So take a little Insurance out, and use roofing screws with the rubber grommets just tight enough to hold the panel in place. With slightly oversized pre-drilled holes. That way the panel can move with much less stress, will extend the life somewhat. Or, bite the bullet and pay good money for the polycrylic panels designed specifically for Greenhouse use. They have additional UV stabilizers.
