Wow, another mudcatter who goes to the musical instrument makers forum. I always feel a little out of place over there, I'm not a pro.

How about getting a big 5 gallon plastic bucket--they are used to hold everything from paint to laundry detergent. You can cut the bottom out if you need to (congas are open ended are they not?) They also have flanges around the top which might help anchor a head-stretching mechanism (lacing or the like). And they even have tapered sides.