How would you build a well cover for a dug well?

First and foremost, "sturdy" matters more than "light" - in fact, a "standard design goal" for a standard well cover is that you should need at least two people to remove it - helps prevent Timmy from popping it off so Lassie can go get someone to rescue the idiot boy again when he falls in the well, again. That poor dog...

Likewise, if a fat person or whatever your largest local wildlife is (so if your bears are Grizzlies rather than Black Bears you need to plan for more weight) decides to climb up on the cover for a look around, it needs not to collapse and dump them in the well. Nothing like rotting meat to spoil your water supply.

So, if going for a "flat" roof (please actually give it at least 1/4" per foot slope) you want something like a small deck. It's only 4 foot span (or perhaps 2 foot 8 inches if 4 feet is the outside dimension and it's 8" block) and "it's not a deck, so it doesn't have to meet actual deck codes" but it does have to hold up a person, bear, whatever. So you can use 2x4 framing, or perhaps skip framing if it's only a 32" square hole, but at least choose thick enough exterior or marine-grade plywood to unquestionably support your largest load with some safety factor.

An alternative is to use a "non-flat roof" as they are easier to waterproof effectively, and may be less attractive as a climbing surface. If 4 feet is the outside dimension, a "5/12 pitch roof" only needs to be raised 10 inches in the middle (2 feet of run each way), and will shed water far more effectively than a flat roof.

You can also go all the way to a "well house" built on top using the well as a foundation, but I suspect that's not a direction you are interested in.

Any well cover, particularly if it's "tight" (as it should be to keep critters out) is normally vented (with screened vents to keep critters out) so that air can get in as you pump water out.

This doesn't need to be complicated or expensive.

1. Build a frame of pressure-treated 2x4 or 2x6 on edge. This should create a well-fitting ring around the well structure.

2. Lay 2x6 or 2x8 boards across the ring and screw them to it.

3. Cover the whole surface with heavy gauge aluminum utility sheet. Lap a few inches, run a bead of silicone caulk between, and nail or staple it down. Hem the corners to fold onto the sides of the frame.

I don't consider a slope in this little thing important. Aluminum or galvanized steel sheet will last many years regardless. If you do want slope, add a 1-1/2" shim under one side of the frame.

Here's what I'd do. I'd make a square of 2x4's standing on their edge, the size of the block. I'd put one 2x4 across the middle as a joist.

Then I'd make a square around those with 2x8's, set up with the tops the even with the 2x4's. That way the 2x8's overlap down the outside of the block wall.

Then I'd put 1x decking on the top.

I wouldn't use pressure treated lumber for this, since it leaches toxic chemicals. Ideally I'd use a naturally weather and rot resistant wood and treat it with linseed oil and / or tung oil. Pine really doesn't hold up too bad treated this way.

Finally I'd make a galvanized or stainless lid to go over the top, and put that mini-deck on top of the sheet metal lid.

If the sheet metal isn't going to happen, I might put plywood on top instead of decking.

If you want to secure it to the top, I'd drill a hole near the bottom edge of the 2x8 on two opposite sides, and sink a big tapcon bolt into the block.