but I worry that there’s not quite enough cover going on.
That’s what bystanders are for!
I bought a box of Prieser 1/100 railroad figures (like, 190 or 200 count for $50!), which are very slender compared to 15mm figs. I left them plain white plastic and mounted them on thin clear bases so they could be used anywhere. Only actively participating figures are painted for ease of I.D., so the faceless masses are just that, quite literally.
"I saw this in a cartoon once, but I'm pretty sure I can do it..."