Looks like no other tricks.
They are pretty straightforward.
The square nut rides in a sheet metal shell (housing). The housing is attached to the engine housing on two sides, and as Peter says, there is a "lip" on the inside (accessible from in the engine opening) that can be bent out, and the nut removed.
You can do a couple of things...
1. Before you remove them, run a tap through them, and clean them out. Run each of the screws through the correct metric die.
2. Remove them, and replace them with square nuts from Ace Hardware, with standard metric screws. Ust black rubber faucet washers on the outside (onto the cover) to give a vibration free engagement.
3. Make a new gasket for the cover opening with high quality weatherstripping.
4. When you reinstall the nuts, squirt in a dab of silicon RTV into the pocket where the nut is captured by the square frame. Then immediately screw all the cover screws in. The silicone will position the nut, but still give you a little "play" so that you can move them around for screw alignment. Don't forget to bend the lips back into place.
It's all easier if you engine is removed.
It's not rocket science. It is about the most straightforward construction on the car!