Best way to heat up my growroom

Hey peeps

I have an outdoor growroom and its insulated well but with winter coming up i want to know what is the best was to raise the temp.

I have 1 underground heat wire heating up the sand but its still at 17c

i want it to be about 24c - 78f

the box is

1.25m wide x 1m long x 1m high

Thanks

DEEJAY

a heater hooked up to a thermostat so it kicks on when temps start to drop.
just be sure to watch humidity levels as well because heat will kill them.

your gonna run 2000w heater in an outside room ?? y outdoor

its where my grow box is. The box has 3" of insulation but when it gets to 7c outside i need to keep it 24 inside.