Frame is an old Kona Cindercone 1997.
Im actually considering something DIY as Im quite handy with metalwork.
I would like to run Hope mono minis.
In that case I'd suggest doing what I've done with my '95 Explosif and that you make up a proper braze on caliper mount.
It's probably an easier job than trying to fabricate some sort of bolt-on arrangement.
You need to make up a fixture to locate the inner face of the bracket in line with the inner face of the dropout and the two caliper mounting holes (at 51mm crs.) at the correct distance from the wheel spindle centre.
Some 10mm threaded bar, some M10 nuts and a piece of 6mm thk. steel plus a few washers is all you need - just make sure that it all ends up at 90 degrees to the wheel spidle axis.
are the IS standard dimensions.
And a photo of one that I made earlier....
A couple of bits of advice - although the inner face of the mount is theoretically in line with the inside of the dropout, it's best to position the mount outboard by maybe .1mm/.004" (space it out during brazing with a couple of caliper shims) and make the final adjustment with shims, because if you have the mount too far inboard then the only adjustment you have is by facing material off the mount.
Likewise, it does no harm to have the spindle centre to hole crs. dimensions 0.5mm/.020" over those specified, as otherwise some calipers (Magura Marta for example) run extremely close to the disc periphary.
Silver solder the whole lot in place with 842 alloy (610-620 deg. C)
You can get the silver solder and flux from these people
The seatstay/chainstay brace is just a piece of tube cut from an old frame.
You'll need to birdbeak the ends so that they fit really snugly - the same with the caliper bracket, it all should fit as well as possible before silver soldering.
If you need any more info just shout, but after this Saturday I will be away from (this) home for seven weeks (with no internet access), so I'll be incommunicado.....
I'll have to ride bikes instead