I bought these for a build and they work very nice. I got one for the radiator over flow and one for the windshield washer fluid...
https://www.speedwaymotors.com/Speed...Tank,7302.html
I did not get them from speedway, ebay I think, but you get the idea. One of these might open up some other possibilities. They come in alot of sizes.
I love the look of your engine bay btw!