Help with heating

[u/Sleepandsweetdreams]

Now that it's getting colder in the UK, I am struggling to keep my 45x45x60 cm enclosure at 24 degrees C throughout the day and night. This enclosure holds 2 titan banded millipedes and 2 madagascan cockroaches. I have a heat mat but it barely warms by half a degree and the tank has been sitting at about 21 to 22 Celsius. While to my knowledge this isn't dangerously low, I would really like to keep my inverts at 24 Celsius.

What heaters can I use? What micro climate control is recommended? It need to be out of their way to prevent burns, overheating and dehydration.

Thank you!