Not sure what type of product you're designing this around , but from your drawing it would look as if it would be some type of fluid that you didn't want to restrict the flow where the additional piping inters. You could take an idea that is used in household plumbing. If you where to make a "break" just up stream of the second inlet that would allow you to assemble this type of manifold with the distance between the inlets adjustable by simply connecting straight pieces of pipe with fittings designed to aid the flow.
I was going to suggest a glass container be used. As Michael Smith mentioned the plastic is a little fragile on most containers that are air tight. With a screw type lid you would have a better chance of storing the paint for future use or touch up on the model you are working on.
When you said that the engine was all chromed I thought of the Ford big block from AMT's '32 Vicky. Looking at the valve covers they seem rather narrow to be the Ford engine but the intake manifold does resemble the Ford intake. Maybe with a little more information such as the exhaust port configuration and the location of the oil filter or oil pan sump location and details about the water pump and fuel pump might make it easier to identify.
I'm with you on this. Going by the model shop today, I'll let you know if I find any. I know I'm not in your area, or country for that matter but if it's out here there is a good chance that it will be out elsewhere soon.
Looks like an interesting project. Unless you are 100% sold on the front wheels/tires, they look a little disproportionately large compared to the rear tires.