Kits for making a spheroid-stabilizing hydrogel in a calcium-free or calcium-chelated cell culture media include (a) a gelation agent including a polygalacturonic acid (PGA) compound or an alginic acid compound, wherein the PGA compound includes at least one of: (i) pectic acid or salts thereof, or (ii) partially esterified pectic acid having a degree of esterification from about 1 to about 40 mol % or salts thereof; (b) a crosslinking agent, wherein the crosslinking agent includes a salt of a divalent ion; and (c) a proton donor, wherein the proton donor includes lactones, esters, or other compounds that hydrolyze in aqueous solutions to form acids over a period of from 10 minutes to 1 hour. Resultant spheroid-stabilizing hydrogels and methods of preparing the same.