Tom,
You can skip the lock.
If you're in the US, NFPA 70 (National Electrical Code) requires that junction boxes have secured covers, so the electrician will secure (drill, tap & machine screw closed) the cover on the old panel for you. If my brain remembers correctly, it's at Article 314.28(c).
In rare instances the new bending space & conductor fill requirements, or metal thickness requirements, don't allow the old panel to serve as the junction box. If so, the electrician will replace it with a suitable pull box. (They are inexpensive.)
A suggestion, submitted with respect, is to have the electrician obtain a permit and have the work inspected. This way, should there ever be a fire, your home owner's insurance company won't have an "escape". With electrical, the adjusters blame anything- cause or not. Then you have to spend money to prove them wrong. I've been hired many times as an expert witness in arbitration cases. It gets expensive quickly. A permitted & inspected job stops this instantly.
You could also ask about costs for an upgrade to the existing load center. It will be more costly than your good plan, but will allow more space for future circuits.
Please Enjoy Today!
Paul
