There's an app on Setapp called Unite, which I tried using to create Site Specific Browsers, or SSBs. I tried it but didn't like the size of the resultant web-apps.
Today I noticed another similar app, also on Setapp, called Coherence Pro. It allows for much more control of web-apps it produces, So it looks like I'll be converting some of my Fluidapp apps over to Coherence Pro ones.