Yes you're absolutely right, it is harder than I made it sound. But not really all that hard. As said, you can copy an existing gaiter to get the fit. Or you can use some cheap material and fiddle with it and adjust it until it's right, then copy that.
What material do you want to use? I can work with cordura on the really cheap lightweight sewing machine I use. Sometimes the motor bogs down a bit, but no worries. The stitches on a gaiter aren't weight bearing, they just need to hold the pieces together and be reasonably durable. I've never had a store bought gaiter fail at the seams. The problems I have had are with the closures.
Paying someone to custom sew a gaiter would be expensive.