I usually do this the hard way. Clip off each models base, make a sanded and painted 100x40mm mdf base (for a troop of Infantry) and fasten each model with superglue and often brass rod pins as well.
This time I thought “lets do this easy”.
I glued the models to the mdf base in the desired formation. Then I glued small cork pieces to “block the exits” of the glue I would use in the next step. But first I let it dry overnight.
The next day I poured in a thin and controlled (ish) stream of glue from top down to fill the gaps between the model bases. Then I sprinkled some sand on it to create some texture. I will wait for it to dry and use some tiny amount of spackle in some areas on the edges where the gap between old plastic base and cork is very visible.
All glue used is simply pva glue.
Photos of result before spackle and paint: