how to make it stay in ?

Turn a groove on the section that fits down the tube, and fit an O-ring. Use a fairly chunky O-ring and play with the groove diameter to get a good grip. We make wheel centre caps using this retention method. The section that fits inside the hex can be cylindrical - no need to make a hex. The O-ring will grip on the flats in the hole

Dave

That's what I used to do, made loads of alloy, brass and stainless frame plugs for bikes and fixed them in with O rings.

The O-ring solution is probably the best way, but I do like Michael G's suggestion with the magnets

Rob

That's how I would do it too.

A 'loop ' pin in each plug with a 'rubber band' through the hollow axle .

I like this one - easy to make and fit, won't damage the bike, and unlikely to get stuck or corroded in place! I'd worry about finding an O-ring just the right size, but I've no idea what's available.

Dave

O-ring sets are cheap, and well worth having if you do this sort of thing.

Or you could glue a custom size one together, as this isn't a critical sealing job

Google Harley Axle Plug and see how the pro's do it. Good healthy spigot sticking 3/4" inch or so into the hollow axle, with an o-ring to hold it in place. If they rattle a bit loose, a dob of silicone sealer holds them in place but allows dead easy removal. **LINK**

Doh, now I feel daft - that's so obvious!