If you make your own, be sure you include fitting holes for all the lamp "standards", often two or more per manufacturer. For instance, Busch und Muller, here known as BUMM, a manufacturer of lamps much favoured on expensive bicycles, offers lamps with fittings at 50mm and 80mm distance that I have, and may offer other bolt separation centres as well.
My fave rack, the Tubus Cosmo, offers only 50mm rear lamp mounting and it is practically impossible to drill the plate for 80mm mounting because it doesn't extend that far, and the construction of the rack is such that you can't bolt on a wider plate either. All this beside the fact that it is anyway a huge pain in the posterior to drill stainless steel in a proper mill fitting under a pedestal drill, never mind in between a bunch of small diameter bracing tubes. I used tie wraps to tie a second rear lamp to the rack; that worked well, but causes "real cyclists'" mouths to turn down at the corners.
Choose your rear lamp before you choose your rack, so that you can be certain to be able to bolt one to the other. Safety first.