the 142 endcaps are for 15mm axles, a shim/reducer would still be required?
Yes, if your through axle is 12mm you'll need a sleeve, but that's much the simplest part to make. There's plenty of 15mm OD 12mm ID stainless steel tube on ebay
Bear in mind that it won't be precisely sized, "15mm" nominal stock usually needs reducing a bit to go through a 15mm hub bore. Given that it's just a rough shim, you should be able to do that at home
Yes, if your through axle is 12mm you'll need a sleeve, but that's much the simplest part to make. There's plenty of 15mm OD 12mm ID stainless steel tube on ebay