I went for a ride with the 1 gallon tank on the front rack. I'm not sure I like it. It might be overkill. It totally changed the bike handling with any more than a quart in it. I think I'll take the stem riser off and just run it like this for the summer, until I figure out a different way to run more than the weeder tank and not spend a bunch of cash.
I have found a couple of go-kart white plastic tanks I could use, if I forego the cargo box (sad), and have thought about various dual-tank set ups, including a 1/2 gallon Toro box shaped tank from a snow-blower.