I have a bearded dragon tank with a dual heat lamp hanging from a metal arm/hook. The dual lamp would point in undesired directions due to the  pressure put on it by the power cords and because one of the “light bulbs” is a ceramic heating element that is much heavier than a light bulb. I designed this simple block to keep the lamps pointing straight down. It fits snugly onto the hook and onto the lamp hanger, as shown in the photos. I also included one photo without the block, showing the original, unstable setup. 

Note that I bought both the dual lamp and the hanger from PetSmart in the USA. I think they are ZooMed brand. I designed the model using TinkerCAD, so all I can share is the STL output file.