'afternoon,
A lot depends on the material and the machine. I had a buddy accidentally start a fire on my brand new wedge tent, resulting in a hole. Tent was the 10.38 oz Sunforger, and the total patch size was going to be about the size of a sheet of paper (allowing generous overlap and "squaring" the patch off). Since it was right near the bottom edge, I did it myself, using a generic (non-commercial quality) machine and regular weight polycore thread. I didn't have to cross any existing seams, and I patched it with the same weight canvas.
It was not fun, and the machine was straining. I ran two seams at the edge of the patch, and one (possibly two, I don't remember) that were at the edge of the hole. It's held up well, and most folks don't even see the patch unless I point it out.
If you're going to put the insert further "in", you're going to have to either hand sew it, or make sure that the machine has the clearance to allow you to work as far in as you want (canvas is bulky). I'd also use at least a heavier machine - I now have one rated for light leather, but haven't had to try it on tent weight canvas.
Hope that helps,
Calum