Here is some pics of a fossil sifter I use here in the peace river in Arcadia Fl. I welded it from galvanized steel and the 1/4 hardware mesh is held on by copper wire ties. Works great, found a lot with it.
Boy that's a blast from the past!! Used to call them beach carts in the day! You might want to consider adding a couple lawn mower type wheels on the back edge. They help a lot for depth control and it's more fun dragging your cart than carrying it. Also if you cut most of the screen off the top it makes it easier to grab goodies & allows you to carry a bucket to your area. Another suggestion is to make a blade for the front that is way stronger than you think you'll need. Make sure you smooth all edges & welds!! Your shins will thank you! Your cart will work around excellent around fire pits in soft sand! With all the nails etc you'll be able to get all the goodies the detectorists won't want to search.