How would you build a well cover for a dug well

If you are fundamentally opposed to (or trying to avoid paying taxes on) an aboveground structure, you could bury a vault to put at least a small pressure tank in. With use of a "constant pressure valve" or a variable-speed "constant pressure pump" you can get away with quite a small pressure tank (2-5 gallons) but you really can't run a pump without any pressure tank, at least not remotely efficiently (you could, I suppose, put a pressure-relief valve in the line at the top of the well, and run without a pressure tank - the excess pressure would recycle water down the well, but that's a terrible waste of power...)

If you really need water in the winter, bury the whole thing below frost line - one common method is to use 4 or 6 foot diameter concrete well tile (as used for shallow dug wells) placed over/around the drilled well casing to provide the frost-free vault - you can insulate the top and sides for better effectiveness. While you have a backhoe in to dig for that and the power line, dig some trenches below frost line to where you want to have hose spigots, and put "frost-free hydrants" at each one (they have a valve at the bottom, and leak away the water in the upright pipe between uses.) Might as well have the backhoe dig 100 holes for trees while you have it, too.

If not fundamentally opposed to a structure, an aboveground structure (small toolshed for the orchard) above the wellhead might be a less expensive approach, unless the local taxing authority makes it an expensive approach after all (that can happen, and is best to be aware of when deciding.) The area with the pressure tank can be heavily insulated and electrically heated to 45F or so if you really need water to hoses in freezing weather, or you can drain the whole system in the fall and start it up again in the spring if you don't really need to use water from hoses in the winter.

If your water does not freeze up in the winter, they are presumably buried below frost line for your area - which is typically where they do get put by competent installers (and not terribly deep if there is no frost.) In my area that's 4-5 feet, typically. In most but not all cases it will probably be the shortest, most direct path from where it comes out of the house to the well. The only way to be sure is to carefully dig down and find it on either end, and then use those locations to try and find it nearer to where you might be damaging it if you blindly pounded things into the hillside.

Once you have found a few specific points, be sure to witness them in one or more of these methods:

1. take very careful tape measure distance readings to fixed objects (the well-head if it sticks out of the ground, the corners of the house foundation, other objects likely to stay put for a long time) and write those measurements down on a map - you can then construct a triangle to find the pipe again by measuring from a fixed point as recorded on the map and making an arc, then doing the same from anther fixed point - where the two arcs cross, the point you measured the well-pipe at should be below (within some scope of error.)
2. I'm fond of a more direct approach, where I construct an annual flowerbed or easily movable perennial plants flowerbed as a physical witness to the item below. You'd still want to save a map with the "important house paperwork" and hope it doesn't go missing, but there's no doubt where to dig unless someone obliterates the flowerbed in question. If you remember not to move it, some sort of lawn decoration could serve the same purpose, but a flowerbed is more lasting.
3. A section of inexpensive PVC electrical conduit just shorter than the depth to the buried item and marked (both what, and from 0 at the bottom to how long it is at the top as a measuring device) with permanent marker can also be buried in the hole as a guide to future digging. The idea being that you can find it quickly as it's just barely buried, and then follow it down and know how close you are to the buried item.