We used a generic, £15 underbody gun with lance - this was attached to an underbody paint tin (with paint disposed of, cleaned throughly, and filled with waxoyl). This is a very easy method of working with the larger cans (5 litre can in this case). You just throw the cheap threaded tin away at the end. Some guns are touted as fitting the 1 litre waxoyl tins with threaded necks so you could do some with that.
Lance is great for getting inside chassis. It's very, very messy but quite easy, just take your time and work methodically. Looks great when finished!
Clarke and Sealey both offer guns designed for underbody spraying with detachable lance.