Pictures are ok but you should ask the cowboy pushing thr cart if he or she would change anything about their cart.
Everyone has thoughts as to what works for them . Try not to overload the cart,try to keep it simple.Some mods stop you from folding the cart,I have used aluminum tubing and angle to keep the weight down. Lawn chair covers make good gun cart covers for rain .Simple wood boxes or plastic containers keep things in order. It sounds like you have a few converted carts to look at .Take the best ideas from those carts . Plastic tool boxes come in all sizes , most are waterproof . I used a plastic automotive battery box bottom for the base of my long guns very tough and lite weight.