Advice on building a pond.

[Ducksareamazing]

I have eleven ducks. 1 pekin, 2 white mallards, 3 indian runners, and 5 welsh harlequins (7weeks old). I have dug a small pond about 5x6 it holds about 65 gallons of water. But It fouls up extremely quickly (within 2-4 days) and start to stink really bad. I've heard that a filter system would work but I do not have any electricity nearby and would have to run some extension cords. I know the pond needs to be alot bigger. But how much bigger? How do I solve the smell issue? How deep should it be? Should I invest in the filter system or can I survive without it? I've got a fairly good plot of land so building it bigger shouldnt be too big of a problem. Thanks!