I didn't have a strap lying around and didn't want to cut up the leather to make one, so I took an old belt and turned it into a mandolin strap with minimal hardware.
It's detachable on one end, can be sized to fit anyone on the fly, and is sleek.
Step 1: What You'll Need
1. Leather belt (any size-my length turned out to be almost perfectly 38 inches, hole to hole) Usually they are 1/2 inch to 3/4 inch, but mine's about an inch and it's served me well.
2. Basic hardware: be creative.
attachment method: this will be the ring that can connect the head to the strap. I used a 1 inch diameter ring, but you can just use string if you prefer
pinch mechanism: I used a nut and bolt, chopped down with a hacksaw to size. be sure not to bung up the threads.
3. Tools: a knife, hacksaw, wrenches if you need to tighten hardware down, bolt cutter for the ring, vise grips for bending the ring back in to where you want it. I also used a punch to widen the holes for the hardware, but a knife could do the trick.
Step 2: The Detachable End Thingy
Take the part where the buckle is held, using a knife, and turn it into the flat leather. there may be some rivets, which you can saw through quite easily. There's a nice little notch where the buckle clasp fits through, and it's perfectly sized for the mando cone. If you need, you can use a knife and cut a notch. Be sure to cut the notch towards the short end, as it's held together by the weight of the mandolin/tension from your body. Use a knife and cut any excess material away. Be sure to leave enough to be sturdy.
Step 3: The Rest
The holes in the existing belt make good places to attach hardware, when you fold it, as pictured. It is adjustable to whatever size you need. I measured out 38 inches, and it worked fine, but can always go shorter.
Once you've got the belt folded over, you can cut off the excess material and use it for your attachment. Cut a peninsula into it that's about 1/3 the width, centered. Punch holes that are the width of the ring. Cut the ring with the saw, and use vise grips to bend it slightly, to thread through the holes. I thinned down the leather a bit to slide inside the strings more easily.
Get the vise grips back on there, being careful not to ding the mandolin, and bend the ring back into place. It'll hold and look continuous once it's in place.
It's that easy! Adjust the size with the placement of the bolts: align whatever holes you want to get the best fit.