+1 for rails like these. I’ve used 10 series aluminum extrusions in a roof rack, roof top tent, awning and solar panel setup. I purchased all mine and accessories from here

https://www.tnutz.com/product-category/10-series-extrusions/

Incidentally, as a kid I loved Fischertechnik kits; they're like Lego but instead of a brick as the basic element, they have X-linear rails.

One thing to keep in mind with this stuff is that it's really heavy. Regular aluminum square tube is much lighter for a given length/size. If you're making something that moves, it may be worth the effort to grab some plain square extrusion and hand-fabricate some brackets.

For a larger project, consider buying from Alibaba. ~5 years ago I built a series of workbenches using 20 series and even with the shipping from China costs, I saved 2-3x over buying from the maker places. IIRC, final total was around $700.