Hi Guys,
I made one of these before but I've lost it, so I made another, this time its a bit more refined (the other one claimed a drillbit)
Basically I found that there were certain spot welds I couldn't get to with an ordinary length bit, for reference I use these cheapies off ebay at 8mm dia.
http://www.ebay.com.au/itm/New-8x80mm-HSS-Cobalt-HSCo-Spot-Weld-Welder-Cutter-Drill-Bit-Free-Postage-/381224615565?pt=LH_DefaultDomain_15&hash=item58c2c1fe8dThe extension bar is from a spade bit set you can buy from Bunnings for around $12. Its a little undersized at around 7.5mm diameter so you could either track down the right size spot weld bit or do as I have and bore it out (ordinary hss bit and plenty of cutting fluid, use 8.5mm to give yourself some clearance, 8mm will do but it's extremely tight)
http://www.bunnings.com.au/craftright-10-piece-spade-drill-bit-set-_p6360080All up assuming you have the necessary 8.5mm bit it will cost under 25 bucks for a 300mm extension bar and spot weld bit. handy for getting behind wings for inner A panels, down the side of the seat cross member (so you don't have to drill from beneath) and a host of other tights spots.
If you get the chance I'd also recommend upgrading the set screws as the supplied ones are usual Chinese rubbish and get chewed out too easily.
cheers.