But I'm wondering if there is a better low-risk option for that money, which is just sitting there for a rainy day, that would help it grow on its own over time and allow me to withdraw funds as needed.  

I’m attempting the BRRRR strategy but can’t refinance for another few months due to having low equity in home.

I personally wholesale lease options, so I can work with low/no equity leads, and recommend everyone learn how to do that too.As for other marketing.

Like, super low.

This is just a theoretical that popped into my head, but for those markets where rents are relatively low, would it be reasonable to take on a tenant with the typical one month's security deposit, but in addition you add $50 or $100/month to the rent to build up the deposit till it hits two month's worth?

There shouldn't be any money due from you unless the fee the seller's agent pays is unusually low. 

@Jon RolandHave you looked into Brandon T's book on investing investing with no and low money down?

Hello BP,My brother and I are planning on purchasing a multifamily with an FHA loan to house hack. My brother has a constant monthly salary (web developer), whereas my monthly salary is more random (restaurant server)...

If your considering raising rents by a large amount, maybe you should wait until the end of the year as an eviction could possibly push you into winter, where your pool of potential  good tenants will be low. 

@Nathan G. the leasing companies have driven equipment and installation costs so low that the systems are relatively cheap for them to install, and they sell the tax credit to entities looking for a tax haven, so that helps them recoup most of their initial investment and selling energy is gravy on top.