sand or salt can be used it goes into many puttershafts too

Would it add much weight. Enough to be beneficial? And would it erode the interior of the shaft?

depends how much you need ? but usually yes - you only need a small amount of weight to notice a massive feel difference when the club is swung

Im a new member to the site so im a little slow to catch up, but try this, i bought a sand iron from a car boot sale £1, drilled an 1/2 inch hole in the blade and fastened a nut n bolt to it, certainly increased the weight enough to swing it slow n easy

Stickman/M3 - The Leaderboard product is on ebay at £149.

http://cgi.ebay.co.uk/LEADERBOARD-THE-ULTIMATE-IN-GOLF-TRAINING-SYSTEMS_W0QQitemZ120069439875QQihZ002QQcategoryZ14109QQssPageNameZWDVWQQrdZ1QQcmdZViewItem