In the preview section of your template you would normally use <flux:widget.grid />
.
Instead:
1) Copy over the files from flux / Resources / Private / Templates / ViewHelpers / Widget / Grid /
and flux / Classes / ViewHelpers / Widget /
to your extension.
2) Rename inside everything you copied to fit your vendor prefix / namespace.
3) Include that new namespace inside your row template and use <YourNamespace:widget.grid />
for the preview.
4) Now you can customize the templates you copied over so that they are floated instead of in a table.