For the RV12 built as ELSA you do not have to keep a conventional build log to prove you built it and the 51%.
What many have done is to simply initial each step in the plans when you complete it and sign and date each page when every item on the page is complete. I would put a sticky note tab on any page with an intentionally skipped task. The plans themselves are your best build log. Also, you can take pictures of the build that clearly show you doing various tasks.
A simple excel file will let you track hours. I kept one of those where I wrote down my hours each day, assigned against the applicable sub-kit. Not that it was "needed," just wanted to keep track.
Some people do a build BLOG with every single page of the plans tracked. Lotsa work to do that.