Place your enclosure to the side or beneath your heat mat. Aim to use a mat-stat for safety and temperature accuracy. Use a thermometer to double check your mantis is getting warm enough and not too hot in accordance with its species requirements. You can heat multiple mantis on long heat strips.
I have an Ikea Billy bookcase and lined the shelves with heat strips; combined with braplast and fauna boxes...it’s pretty neat.
Table lamps or lamp holder/dome and canopy units suspended over the enclosure. A normal incandescent or reflector bulb is usually enough. Be aware that LEDS will often not produce enough heat. Experiment with different wattage until you create the desired temperature at the top of the enclosure. If the mantis gets too hot they will move to the cooler lower areas of their cage.
Do not to rest bulbs and hoods directly on fabric mesh enclosures as they can pose a fire risk.
Note: No UVB or special lighting is needed for praying mantis.
Most of us nutters...I mean exotic pet keepers have reptiles who have heating in their vivariums. The area directly above the basking bulb in the vivarium can produce enough heat to warm your mantis too. Some of us have multiple reptiles and the ambient room temperature is significantly higher than a regular persons' room temperature. Those who have reptile rooms will often not need external heat sources for their mantis but everyone should check first and check throughout the year.
Yes your orchid mantis will live at 20c but will it thrive? We want to keep our mantis happy and healthy so always aim to provide the best care and part of that is adhering to their temperature ranges. If in doubt always check your temps with a thermometer or temperature gun.