My next question is about the counterweight. Unfortunately no pics but I tacked a piece of angle at a 45° to the center of the lid, and measured out 1/2 the width of the lid to find the equilibrium moment, or some fucking thing I'm not an engineer, of the weight of the lid. IOW if you put 165 lbs (lid weight) at that spot the lid would, in theory, be in balance. But of course you don't want that, you want the lid to stay closed, and open. So I just picked an arbitrary amount to place my counterweight completely above that point.
My counterweight of choice is a forklift propane tank. I didn't post this is "shit you heard in a bar" but a drunk redneck from Tyler I was talking to at my local place swore up and down that a small propane tank filled with sand would be sufficient for even a 48"* 500 gal pit.
So then I calculate the weight of my lid (165 if I recall correctly), and calculate the weight of my forklift propane tank filled with sand, and decided I'd better extend it out horizontally for more leverage because it was only going to be like 95 lbs. full of sand.
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tldr
I guess my question is how the fuck do you design/position your counterweight.
* Also wedded to a 48" door as this thing is meant for whole hogs.