So far as I know, temperature has little influence over the sex ratio in guppies. They naturally produce a higher number of females to males, because in nature, females are worth a lot more in terms of overall survival. One male can fertilize a number of females, who can have a number of fry.. so evolution has decreed that more females are more likely to keep the population going.
You're fighting an uphill battle. The only way I can think of to change the ratio much, would require a lot of observation. You would have to know what fish fathered what fry on what female first. Then you'd have to count the number of males/females in each batch of fry, which means waiting 'til they are old enough to sex, for starters. Then only breed the males who produce the greater number of male offspring.
It is males who govern what sex offspring will be, so you need to select your male stock for their ability to produce more males. I can't imagine that is even going to be possible breeding them in a big cement tank.