I used Pine lengths about 110mm wide and just cut them down to the typical depth of a roll of toilet paper. I did some rough dry fitting to find out how many rolls would fit depending on the size, and came up with a measurement that looked about right relating the overall size of the wall which was going to hang.
The shelf was trial-and-error. There is a formula for cutting a shelf with beveled edges on it so it fits diagonally flush to the corners. Once it was made, I just used some timber stain and brushed it and then used a old rag to spread the stain out and thin it out enough to see some of the original Pine.
To attach it to the wall I used wall plugs and two long screws. It's not great, but it's still sitting on the wall no problems. In hindsight, I could have secured a small batten to the wall and then put screws down through the shelf part and into the batten.
How to build a toilet roll holder shelf
As this has been an extremely popular project with the Bunnings Workshop community, resident D.I.Y. expert @MitchellMc put together a step-by-step instructional guide complete with a full list of tools and materials. Check out the guide at How to build a toilet roll holder shelf.