In-car phone mounts are a dime a dozen, but they're all a bit clunky, especially in a small Miata. Suction cup on the windshield? Glue on the dash? There's a better way! These pieces replace one of the "eyeball" vents (any of them) and give you a place to connect a RAM mount. RAM mounts open up a world of different mounts. Basic arms to hold a phone? Wireless charger mount? Lap timer mount? It's up to you.
There are two versions of this mount, recessed and proud. The proud mount has the RAM mount proud of the vent, which allows for lots of freedom in where the mount ends up. It's also more expensive, bulkier, and pushes whatever you use farther away (rearward) from the dash. The recessed version is the opposite - more limited in where the mount ends up, but a cleaner install and closer to the dash. The recessed version might require that you replace the thumb nut with a normal nut (1/4-20 nylock) for clearance reasons. Check out the pictures for more details.
Bear in mind that both of these will move a tick, as the car goes over bumps, but nothing at all unreasonable. While these are very strong, there is a limit - these should work fine with a smaller lap timer, but, might not survive a larger and heavier one driven on a rough (e.g., lots of curbs) track. This also applies to adjustments. The attachments can apply a lot of leverage, so be sure to fully loosen the arm before trying to adjust its angle.
You'll need "B size" accessories, which you can find here. The shortest arm possible is probably going to be your best bet.