If you're going to make this tool, I'd advise the following:
1. Take your own measurements of your camshaft ends using a vernier caliper - preferably before you remove the chain and sprockets. The pic above was published in the Haynes manual, and the dimensions aren't that accurate. If you add up all the widths along the bottom of the diagram, it totals 241mm, not 240mm as it shows at the top of the pic.
2. Use steel plate, min 2mm thickness. Otherwise you'll have trouble keeping the tool in place, due to the camshafts trying to rotate under the force of the valve springs.