Unfortunately this is a bit tricky but it can be done
TML
---
<t:block id="myBlock">
<t:cardLightbox cubeId="cubeId"/>
</t:block>
Java
---
@InjectComponent Block myBlock;
@Property myBlockBodyAsString;
Block beginRender(MarkupWriter writer) {
writer.element("container");
return myBlock;
}
void beforeRenderTemplate(MarkupWriter writer) {
Element container = writer.getElement();
myBlockBodyAsString = container.getChildMarkup();
container.remove();
}
Then you can reference ${myBlockBodyAsString} in your template.