Pretty simple really.

Does anyone know if/where I can get some flat-style SPD shoes?

Something Similar to 661 Launches or Shimano 66 DX's, but in Size 15, at a push size 14 will do, but no smaller. Do any companies do custom orders for this type of shoe? Does anyone know what size the new Vans SPD shoes will be available in?  Cost isn't a consideration, just so long as they fit.

Failing that, does anyone know what parts/materials I'll need to bodge the cleat mountings from Shimano MO21's to a pair of Vans Old Schools? 

That, Mr Riley, represents my unamused face, it is not amused...  

...unless of course, there was a serious point in their somewhere. 

I have size 15's as well your only option really are SIDI Dominator 5 these are made up to a euro 52.  I wish i could get some winter ones but i cant find any other company that makes over a size 12   as for Vans style almost no chance what so ever

On the plus side my Sidi's last at least 3 years if not more. 

MTBikeMad *The Ice King!* wrote (see)

I have size 15's as well your only option really are SIDI Dominator 5 these are made up to a euro 52. 

On the plus side my Sidi's last at least 3 years if not more. 

Any idea how they would stand up to being regularly downhilled? 

More importantly, how do they fare protection wise and will they take being dragged through the dirt at speed? 

I've tried size 52 Sidi's and they come up small - also size 50 shimanos and size 50 Lakes (they were tiny).

Best were the size 50 Northwave Kaimens but even they (the 2007 model i think) felt tight compared to my 5+ year old Northwaves.

Think i've run out of options now so when the current Northwaves fall apart i'll be using flats on all my bikes