What I've done with other strapping situations in the past, is once I get it dialed in to where I want it, I'll just sew 2 lines near the buckle to lock it in place. If you only do a couple lines of stitching, it's pretty easy to remove. Now, granted, webbing will stretch, but that may take some of the slack out when you get it where you want it.
They also make these tri-glides for webbing which are also used to lock longer pieces in place that may or may not work in your situation.
They look like this...
View attachment 224697
Hope this helps, and great work on your Jango suit!