UE4's built-in blueprint function 'CreateWidget' only allows you to specify UUserWidgets, so to add something basic like a button, border or text at runtime, you have to create a custom UUserWidget to wrap it - very inefficient. Luckily, there's a ConstructWidget() function in C++ that can easily be exposed to blueprints.
Make sure you've got a BP/C++ project, then create a blueprint function library C++ class.
For the island used in Tropical Manipulation, I wanted a simple material applied that I could quickly tweak to get a great looking island. I investigated using landscape layers, but found that for what I was using it for (an unchanging backdrop to the game) they wouldn't be necessary.