This is quite easy, since you can make it permanent for the wearer's size.

	Just sew the loose end of the strap to the other part at the correct length. (Can you see the stitches?) It doesn't matter whether there's a slider at the end of the loose end or not. See also Ariel in Snug Overalls, esp. the picture under the gallery.
	If you use the strap at the extreme length, just sew the end of the strap to the fastener. This can still be done pretty inconspicuously.

With other fastenings, like belt buckles, securing the fastening automatically fixates the strap length, which makes it all the easier.