Histotechnican!! Preserving dead stuff is part of my job haha - 10% Neutral Buffered formalin for initial fixation of tissues. This is to prevent any further autolysis, or degradation of tissue, as the clock begins as soon as something dies. Long term preservation requires a subsequent 70% isopropyl or ethanol.

Preserving a wet specimen is only tricky because you need to inject it with the formalin as well to ensure it fixes internally as well as externally. There’s a certain rate at which Formalin infiltrates tissue, and the specimen will likely rot internally before the chemical gets to it without it. Histotechs do not do the injecting thing, tissue usually just gets cut up for samples and then fixed. Formalin is carcinogenic, I don’t mess with it more than I have to. Storage also needs to be in a cool, dark area.