I used square nuts from boltdepot.com. Here's a list of parts from my order with them (could probably go with shorter bolts for most stuff - wound up needing a bunch of washers to keep them from bottoming out on the mast)
Product #143
Hex bolts, Stainless steel 18-8, 5/16"-18 x 3/4"
Quantity: 12 pieces
Price: $0.22
Subtotal: $2.64
Product #2948
Flat washers, Stainless steel 18-8, 5/16"
Quantity: 1 box of 100
Price: $3.99
Subtotal: $3.99
Product #2957
Lock washers, Stainless steel 18-8, 5/16"
Quantity: 24 pieces
Price: $0.07
Subtotal: $1.68
Product #7736
Square nuts, Stainless steel 18-8, 5/16"-18
Quantity: 12 pieces
Price: $0.46
Subtotal: $5.52