The best way to tackle any problem is to go straight to the source so I'm going to start this tutorial by looking at a real stirrup leather on and off the saddle. This is the buckle end of a typical English stirrup leather. Note that this is essentially a single loop, no keeper buckle.
Here's that same stirrup leather looped around the stirrup bars of my old pony saddle. As you can see, some amount of "bump" is unavoidable, even when the stirrup bars are recessed.
I have two different strategies for making adjustable, model sized stirrup leathers. The first is to mirror the real thing as closely as possible. To that end, I start by threading a single loop tongue buckle onto a prepared length of kangaroo lace. I like to use a 1/8" photo-etched Rio Rondo d-ring for the metal part of the buckle. Photo-etched hardware is very flat which is perfect when you are looking to reduce bulk.
Thread the stirrup iron onto the leather and loop it around your stirrup bar. Tuck the loose end into the stirrup leather keeper and that's it. (I should mention that I don't generally attach the stirrup leathers at this stage of saddle construction. I used some discarded scraps on my workbench for the pictures in this tutorial.)
The second method for attaching adjustable stirrups to your saddle is less realistic, but works just as well. In fact, I tend to prefer the simplicity of its bucklefree construction. I start out by preparing a piece of kangaroo lace, but I do not attach a buckle to the non pointed end. Instead. I loop that end over the stirrup bar itself and glue it into position grainside up.
Thread the stirrup onto the lace and then bring the pointed end up and over the backside of the stirrup bar. Tuck the loose end into the keeper.
And that's it! The saddle now has buckle-free adjustable stirrups leathers. To shorten the leathers, pull gently on the loose end.Here's how to lengthen them. Be careful not to pull so hard that the loose end comes through your stirrup bar. It can be awfully difficult to rethread the leathers once the saddle is completely assembled!No matter which method you choose, be sure to spend lots of time preparing the lace before you start. Unskived lace will not fold neatly over itself and will always result in that unsightly bump. Preparing straps and leather pieces is by far the most time consuming part of tackmaking. It's slow, tedious work and it's tempting to skip ahead to the more enjoyable construction stages. However, if you want to make a quality product, it's important to take the time on the front end. I wish that wasn't so, but...
Thanks for the question, Jennifer. Hope this helps!