If you are winding your own coil, you usually have a circuit you are trying to match it to, either one you built, or a kit that came with instructions and some specifications on the coil. If you are replacing a commercial coil, you would want to start with a working one and reverse engineer it. Got an vector network analyzer available, and a lot of patience for the "alien" autopsy?
A three foot on a side square coil may be what you need if you are looking for a tank sunk into the Siberian tundra... not exactly an off the shelf item.
You may find it hard to home build a coil lighter or more waterproof than many of the available commercial units. 3d printing a coil base and a winding form works for me. The coil can be dropped in and sealed with an alcohol based sealant like used for affixing heads on hard drives, the more common ammonia based sealants may cause corrosion.
PLA (the most common type of 3D filament) is water soluble and will dissolve if left wet long enough, nylon or ABS may be a better choice, but both require a better printer.
If the coil still works, drying it out and sealing it would be a good idea. Try your sealant of choice on a chunk of copper and see if it turns green (corrosion) before finishing your sealing job. I used alcohol based sealants on the solar array for the University solar car, so I do have some real world experience here. You might want to soak and rinse with de-ionized water to remove some contaminants, before baking and waterproofing.
There are videos available online, and pre-wound uncased coils available on on Epay last time I looked for some popular models of detector.
