Another vote for the Speedy Stitcher.
I've not used mine for an SRT harness as I'm slightly nervous of fixing or making something so critical, but I've sewed pretty much everything else. Oversuits, belts, tackle bags, knee pads, vinyl hood and seats for various old cars etc.
I've found the thing to be lethaly sharp and cut my hand pretty badly when I first had it. For thicker materials, I tend to push less hard and give it a gentle wiggle as I push and it seems to go through in a more controlled way.
The downsides are that it's not as speedy as the name suggests if you have a lot to do and winding on the bobbins by hand is really tedious.
I would consider it an essential in any cavers toolbox.