I use the EzyView Rough Transparency Fluid from Adamas Facet: https://www.adamasfacet.com/ezyview.html
It works well for me, and I use it rarely enough that it’s lasted me forever, so I’ve never had occasion to try other alternatives. Tom Smith (owner) says it’s safer than the old Refractol, and I believe him, but I wouldn’t do shots of it 😉
–Eric