I’m not suggesting you change an existing board, I’m suggesting you just use a transistor. When working with low voltage DC you can use components directly without the need for fancy boards. It know it might be scary to work with solder but if I can do it – and I’m a CS grad, not EE – then you can too :). There are lots of examples on the web on how to use it to switch DC. And from a cost standpoint, you can’t beat it – I see you can buy one of those 2N2222 for 10 cents ($0.10).
And besides, if you fry something it probably won’t hurt anybody. It’s when you start messing around with AC 120v 15A that things get … challenging. That’s when buying a board is a good idea – somebody else has already worked to make that safe.
You can pull a lot of mA out of a 9V battery; but that’s not the issue – the issue is the load, so you’ll have to look at the solenoid and see what it draws.
What are you trying to do?