Bank/turn indicator instrument question

I'm considering ripping the curved tube out of an aircraft bank
indicator to use an inclinometer (or clinometer) on a bicycle.

There's a bicycle bubble level on the market but I don't think it's a
good design.

My question is, on aircraft bank indicators, why do they look like they
are about 6" deep? What is in this rear of the instrument? I'm sure
there's a light bulb in there but this does not explain it. Perhaps
there's more to it than a steel ball in a tube?

Thanks for your help.