Great idea for a thread.
1) For a secure, dry, unobtrusive long lens bag I use a 20 liter dry bag lined with closed cell foam from a camp sleeping pad. Two disks of the foam form the bottom of the bag. This bag is big enough for a 500mm f4 w/D850 attached. There are no "steal me" labels on the bag and I've always gotten away with claiming it as my "second personal item" in addition to my camera bag for carry aboard on US airlines.
2) I made a seat out of 3/4in plywood and closed cell foam to fit on a five gallon bucket. I use it as a seat and carry a roll of burlap material for impromptu hide, my lunch, bottle of water, etc. in the bucket. The large surface area of the bottom of the bucket works well for sitting on soft mud etc. I painted the bucket camo colors.
3) Closed cell foam tubing made for pipe insulation cost next to nothing and can be used for a number of things. When using a tripod aboard a boat use zip ties to attach foam to the bottom of the tripod legs to dampen vibration.
4) Similar to the foam insulation tubing but larger is closed cell foam swimming "noodles" for kids. Available in the toy section in most department stores. It makes for a light weight alternative to a bean bag for resting a long lens across the gunwale of a boat, out car windows, etc.
5) Zip ties. Don't leave home without them.