For example:A clear or black (if you don't want anyone to see what's inside) acrylic wall-mounted suggestion box (both of which fit the dimensions you listed and even have a slot at the top for running wires out of):Link to product: Clear Suggestion BoxLink to product: Black suggestion boxOr just a plain acrylic lockable box that comes in various sizes:Link to product: Clear acrylic boxI wouldn't think it'd need to be anything too fancy or expensive.  

Your margin for error would be razor-thin.Moreover, I think you'll feel better about making investment decisions when you have the proper financial tools to analyze your deals.

Although, the sharks will be swimming around this post...Most of us waste money on cars, friends, and the like; so, no worries there. 

My wife and I joke how we could have been thrown in the slammer for loan sharking for our first few deals.

Don’t be a shark.

Seller financing is also an option that should be mentioned, but finding a seller who will finance the renovation (not just the purcahse) is little like finding a unicorn.There are a lot of great hard money lenders in most markets, and there are always a few sharks.

Assuming you guys are using conventional lending, if all three of you were on the loan, then this one property takes up a slot in each of your allotments for 10 loans.  

Could be genius and we will all see you on shark tank next season!   

Could you possibly clean it off with razor style floor scrapers?

This will free up slots for Fannie/Freddie.