Dig a pit and build a fire in it*, or use a firepit with a built-in grate. Let fire burn to medium-high and spread out logs as needed; if using your own grate, set it in place.
Fill a 20-qt. canning pot with 4 in. seawater or salted regular water. Cover and heat to simmering over fire or over high heat on stove.
Wrap potatoes, corn (divided into 2 bundles), clams, and cod separately in cheesecloth: Cut five 30-in. lengths of cheesecloth. Unfold a cheesecloth length to yield a double-thick piece. Lay it on a table, pile the ingredient in the center, pull up two opposing corners, and tie loosely. Repeat with other corners, creating a pouch that's secure but loose enough for ingredients to spread out in more or less a single layer.
Put potato pouch, loose onions, and bay leaves in canning insert, and when water is boiling, lower into pot. Cook, covered, 10 minutes. Add cod pouch, then clams, and top with loose crabs. Cover and cook 8 to 10 minutes more; cod should be opaque, the crab should be warmed through, and clams should open (though it may be difficult to check).
Using tongs, transfer the bundles of ingredients to a newspaper-lined table or into large bowls. Put corn bundles into canning insert and into boiling water; cook until tender, about 5 minutes. Meanwhile, drain off any water from bundles in bowls. Snip open cheesecloth and pour ingredients out onto platters or a fresh spot on the covered table. Throw away any unopened clams. Serve with Tartar Sauce and Italian-Style Salsa Verde.
*The chefs use a grate with legs (Stansport Camp Grill, from $22; stansport.com). A charcoal grill grate set on bricks also works.
Note: Nutritional analysis is per serving without sauces.