Get one fo them Reel-lites from Bob Glen on here (they arent the cheap Wal-mart kind) and use some minners or jig, you can add a split shot if needed to get it down to 8ft.
If you are saying you dont know how to measure 8ft down without using a cork. Use the length of a pole, if you have a 6ft pole hold it up and let the jig/minner down until it is the total length of the pole. Then you can just pull out and guessetimate the last 2ft.
I wouldnt bother with a lighted bobber or any bobber for that matter.
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