We've used gelled agarose to cover & hold monolayer cells in place when treatment leads to cell lifting. Low-gelling temperature agarose (Type VII) (Sigma A 4018 or A 0701) can be made as a 2% stock solution in “phosphate-buffered saline” (PBS) (boiled then kept at 42° C to remain liquid). The agarose can be stored at room temperature at this time, but must be boiled and melted again before use.