To the best of my knowledge-
Templates and their cells (as provided in LR) are fixed and not changeable.
However you can-
Alter the image size and position within a cell by adjusting the padding sliders, or drag the cell margins on the page. (remove tick to unlink margins)
After altering padding , the page can be saved as a "Custom Template"
The second option is to design custom pages in the Print Module using "Custom Package", and 'Print to JPG' the page as an image for use (full page) in the book. Slow and tedious, I know, but much more creative possibilities.