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.  

On top of those challenges it had on site septic and well water.

Without seams, you don't have to worry about water or urine getting under it. 

I ran into that problem multiple times where I put in a bid a few thousand less than asking price that I KNEW they would accept, only to have an out of state investors blow my offer out of the water. 

Just clear the muddy water by doing a straight up 50/50 deal.

This will not cost you anything and you will obtain 95% of your water expenses every month from your tenants.

Currently the guest house is not separately metered for water or power.

If your numbers are accurate you should make almost 40k after paying your realtor :)Here are the numbers to back up my argument that this is not a good rental: Mortgage Rate 3.00% Length of Mortgage in years 30 Monthly Mortgage payment $420.55 Taxes $ 133.33 Sewer and Water $ 125.00 Trash $ - Heat/Utilities $ - HOA/Legal $ - Cap Ex and Ops $ 250.00 Insurance $ 100.00 Mgmt Fee $ 110.00 Vacancy $ 91.30 Total Expenses $1,230.18 Unit 1 $ 1,100.00 Unit 2 $ - Unit 3 $ - Unit 4 $ - Unit 5 $ - Unit 6 $ - Total Revenue $ 1,100.00 Cashflow/month $ (130.18) Cashflow/year $ (1,562.20) Cash on Cash Return -4.03%

I'd love to sell you something in my neck of the world but there's weird in the water and currently it's a bit of a bloodbath. 

I know a lot of people focus on Short Term Rentals over when you get closer to the water, but you need to be careful about what city you're in as STR are a very touchy subject.