Opposing wedges are strong, they’re quick to lock in and knock out, unlike toggle clamps they don’t get in the way of the workpiece and better than all of that…they’re cheap/free!
Hardwood vs plywood/softwood?
For how little material you need, I’d recommend going with hardwood for durability. Softwoods will tend to dent and wear out, and plywood may not actually be as strong — remember, half the grain is going in the wrong direction!
The easiest way to make wedges is to cut them at the bandsaw, but it isn’t overly critical how you cut them. I used an 8degree angle, but again, the dimensions don’t really matter, so long as they fit on the jig.
Cams are better locking alternative?
One idea I have not experimented with is using a cam lock rather than wedges to provide the clamping action. This would be marginally more work to make, but should be easier to operate.