That's definitely one way to do it, but melting and casting is by no means required. A Stirling engine can be built with existing parts of things, especially if diaphragms are used in place of pistons (since rubber and plastic membrane material is common and easier to construct with). Even better if you can find a super robust rubber like from truck air brake diaphragm rubber, but the possibilities are really endless there.

> There is no proprietary tech in stirling engines that might really make them long lasting/ highly efficient ?

Stirling engines are at least as efficient as solar panels, and probably more so. Their efficiency is often overestimated but by default they're relatively efficient.

The difficulty with Stirling engines that no one talks about though is managing excess heat. They're subject to the same issues as internal combustion engines in that they can't convert all of the heat input into work, hence you have to either control the rate of input or you have to use cooling as a corrective measure to maintain a temperature gradient. Otherwise, the engine will stop working. The advantage to powering a Stirling engine with the sun is that the input should be easy to adjust, whether it be mirrors or a lens that respond to a thermocouple.

There's other ways to improve efficiency, none of which is proprietary. There's a Stirling engine variant developed by NASA that's entirely enclosed and uses helium as a working fluid. It's meant to be efficient and robust because there's no crankshafts or direct linkage between the pistons.

https://www.youtube.com/watch?v=P1FwrDZKfKk

One particularly prolific content creator shows how a NASA Stirling can be built with common materials:

https://www.youtube.com/watch?v=dOSGpzGhrJQ

Another way to address robustness is to make a Ringbom style engine (still Stirling cycle) that uses a long tube to make a gas vacuum air piston, thus reducing the mechanism to just one moving part:

https://www.youtube.com/watch?v=_Fz0jsOlL60

^^ That maker refers to it as a thermoacoustic "metronome", but it's actually a form of Ringbom engine.

Then there's actual thermoacoustics, which is a pretty interesting area of research.

Here's an example of a solar powered thermoacoustic pipe:

https://www.youtube.com/watch?v=fMoRkEffFOQ

The downside to thermoacoustics is that they're inherently loud, unlike engines that use the Stirling cycle, but they are less physically complicated than Stirlings. In the above example, the acoustics aren't actually driving anything, but said technology is capable of working a piston like a Stirling does.

A very niche area of thermoacoustics involves using steam as a working fluid to improve efficiency. Here is a video of a "wet" thermoacoustic engine I made recently. This engine is simply two cans taped together, a ~3 cm layer of rice, a balloon with weight attached, and ~100 mL of water.

https://www.youtube.com/shorts/Rh4m8mrB2t8

But it's not a steam engine! When running it performs entirely enclosed without pressure overload.

Thermoacoustics might be too loud, but overall I think that heat engines are underappreciated as energy transducers. Diaphragms are used frequency in amateur toy heat engines, and actually solve a lot of the robustness issues that come with keeping a piston-based engine running, but research usually goes into pistons. A fleet of Ringbom style low-temperature-difference engines using diaphragms and flexure springs in place of crankshafts could probably generate a meaningful amount of power while being low-maintenance. Perhaps they're not as practical as solar panels in terms of energy produced in a given area, but they can be an alternative.

Imagine a rooftop of these except with no crankshaft or flywheel:

https://www.youtube.com/watch?v=PD0gofiEhj0

The only negative being that they would perform more poorly than a solar panel if it's cloudy out and the temperature difference isn't great enough.