The trick here is 4 parts.

1) Being able to tighten the glow plug in the glow adapter.

2) Being able to get an adapter on the glow plug to power it.

3) Getting the glow plugs tip into the combustion chamber, a concaved end on the glow adapter helps.

3) Not lowering the compression raito with the glow adapter.

These make for a pretty deep and thin walled glow adapter.

If you can make one like this I'd buy one......