Rise 360: Does anyone have any tips for design consistency when adding custom interactive storyline blocks please?

It does vary by project.  That being said, I create a LOT of what I call Storyline banners for use in Rise. I typically set my slide width in Storyline to 1000 px and then the minimum height required for whatever is on my slide.  Sometimes, this is as little as 300 or 400 px. 
On the Rise side, I typically use 0 to 10 for the padding top and bottom. Depending on the content, I will set the width anywhere from Medium to full width. 

This example only features one Storyline block towards the end but you can see how it fits well within the other Rise blocks.
https://360.articulate.com/review/content/6213e9b5-46d2-4755-9f43-1cf4995cbb49/review

I would also note that I try to avoid "Storyline in Rise" projects that will be accessed through mobile. I've been fortunate living in the corporate world that my users are behind a firewall accessing content on their PCs only.  The block just doesn't look as good through the mobile browser, IMHO.