hi. i did a write up a while ago on the subject, it was kind of lost within another article, so here it is, on it's own and slightly updated, like the bike it's been to hell and back! and is still fine.
if you want one made i can supply a kit with all you require[bolts,nuts, drill bit, ktm block, uk postage] apart from drilling the swing arm and fitting! is �45 ok?
about jan 2010 and 25,000 miles ago i made one. it uses the older ktm genuine chain block circa 2002 it lasts the longest [years of owning exc's].as it lasts so well it wears the split link so riveting the chain is advisable. has survived very well on proper off road!
the bracket is 3mm stainless plate 52x 65with a right angle fold, so now52x40 with a 25 right angle. the slightly slotted 8.5mm holes where it secures to the swing arm.
the swingarm is drlled to the centre of the flat part,the rear most bolt is app 180mm from the end of the swingarm, i used stainless bolts facing down so easy to remove or repair if damaged on the trail. the bolt heads have a foot welded on to stop them rotating in the hollow of the swing arm. the block is fastened with 6mm bolts the rearmost hole is slightly slotted to adjust to sprocket size
i hope this helps, very reliable, easy to make and fit.give a good place for the scottoiler too. any probs give me a shout, stoic