The correct tube size is 16" x 2.125"

The valve stem must be angled, not straight

There are several angles that angled valve stems could have for 16x2.125 tubes.

The correct stem is bent 90 degrees aligned with the plane of the inflated tube

A stem bent 90 degrees and rotated normal/orthogonal to the plane of the tube will catch in the wheelwell

I was able to "cram" this tube with the stem that is bent about 60 degrees and rotated about 30 degrees. I had to twist the stem before and during inflation so that the cap wouldn't click against my wheelwell. It held air for about 300 miles and now I'm changing it again.