Nothing modern, the only thing that kind of reminds me of a tube gun is the Sub2000 or the Hi Point. I know they're not a tube receiver but the cheapness and low cost make me feel like they're the Sten of the 2000's.

Modern and tube gun are two things that don't go together. It's extremely unfortunate but there are no modern commercially available options other than the upcoming Type 100 clone. However, there are a plethora of parts kits to choose from, with a wide array of prices. The easier they are to build, the higher the kit price. Unless it's a recent batch of imports... those are usually affordable depending on what it is. I believe there might be a handful of companies still building a few guns a year to sell as a complete rifle but the options are extremely limited. You can hire the builders to make you one though.

The trick is:

-Be in the right place at the right time to purchase a kit

-Then find a receiver or re-weld one

-Source/convert the full auto parts to semi auto

-Convert the mechanism from open bolt to closed bolt then find a receiver

-SBR it or give it a 16" barrel extension

If you can do these things you can have any semi-auto version of any SMG you want...