How to fill empty space of table (2d array) with random character(C)

I have 10 word.I want to fill them in 20*20 table(2d array,direction is not important)and after i want to I want to fill with random character in every empty spaces.For example 5*5 grid should be like . words are "fast ,car ,how,is" After fill the table with my string how to find location of empty positions where I will put a random letter?(there shouldnt be empty line it is not coding problem i cannot edit this properly)

fastx

carcd

howef

sghji

iklmn