I use a true “kedeira hafucha” on the blech, by placing an upside down 9×13 baking pan on the blech or an upside down pot (depending on how strong the flame is). I find that it warms (“mafig tzina”) but does not reach yad soldes – of the higher temperatures for y.s.. I also find that it won’t melt the sturdier plastic containers (the cheaper plastics may warp a bit). Experiment a bit what works for you.
I am very machmir about warming chicken or kugel that has moisture – I consider it bishul if it reaches y.s.
I am also very machmir on melting congealed liquids (as common on chicken or meats) – which I consider 100% ossur even if it does not reach y.s.
I know that many yidden are meikil on this, it creates a problem when my husband and I are invited to someone’s seuda and the food served was nisbashel on shabbos (according to chumra). People don’t even think about this halacha and simply wimp out on a real chashash chilul shabbos.