I don’t have birthday candles right now but this is what I do:
If you have small jars from votive candles or even a shot glass, pour some sea salt in it and stick your candle in it. Make sure they won’t fall and don’t leave them burning unattended.
Another DIY solution would be making small candle holders out of clay.
@Volmarr here I am talking to myself answering my own question! But I realized, and then a few paragraphs down where I was writting about birthday candle spell use also read what I realized in my mind, that I can use a small bowl of dirt and put the birthday candles in the dirt as a good method of holding them up when using them for spells! Yay!
You’re welcome! Let us know how it went! 
