i would have it divided into 8..........and use clear dividers, they look like the fish are swimming together, and yes they can get attention too........im doing that to 2 20 gallon long tanks that i have
also, you wouldnt need 2-3 heaters, just one rated for a 20 gallon tank will do it.
if u divide into 8 that would mean one long piece 30" dividing the middle.........and then smaller dividers into 8 compartments. that would mean 2.5 gallons each, and a space the size of 7.5" x 6" x 12"
thats enough room for a betta