The thing with hand tools is that YOU are the key part that provides the consistency. When dealing with hand tools not everything will have a stop or dial or setting to make sure it's accurate. The assumption is that the user will be providing the necessary skill to make the fine details consistent.
(This specific task is somewhat complicated by the fact that you're using plywood. Plywood, along with MDF, particle board, and other sheet goods, is engineered to be used with power tools. With hand tools a lot of the small details come down to how you're working with, across, or against the grain. Plywood has grain going multiple directions at once, so it can be difficult to work with it in 3 dimensions.)
Personally, I would just get in there with an exacto knife or the corner of a chisel (bevel down) and make whatever edge treatment you want. Generally with hand tools a chamfer is easier than a roundover, but both are possible. The burden is on you to not cut too deep, but with some practice it will come naturally. In hardwood you'll start on the end grain and move towards the long grain, stopping when the grain reverses. Skew the cutting edge so that the top surface is cut first to avoid tear-out. Take thin cuts and make sure your tools are sharp.
You might be able to put a chamfer on with a curved-sole spokeshave at an angle. I don't really expect this would work, though, since the curve is so tight.