I realize that this is somewhat off topic for your question but 45 lbs is kind of big for a single tank. I assume that by mentioning cold water diving, you are referring to the extra weight needed to off set thermals or a 7mm suit as well as a steel tank. Keep in mind that the wing really only needs to be able to float your rig at the surface without you in it and off set a total loss of bouyancy at max depth (for a 7mm suit that's around 100 ft.). In most cases, 30# is more than adequate but your mileage may vary.
Do a quick search on "Cool Hardware". Tobin from DSS has numerous posts on how to calculate your lift requirements.
DSS, Halcyon and Oxycheq (in that order) all make excellent singles wings in smaller sizes. Most likely, you will find the DSS to be the cheapest. It is also the best design, by far. I have a 40# DSS LCD wing and wish I bought the 30#.