Hi! I'm on a bit of a resonator kick myself, and i'm currently playing with the idea of turning some cheap kid's guitars into Dobros, but i've done a few Ukes, Dulcimers and one banjo so far.
CB Gitty sells actual Resonators and covers in various sizes, as well as the covers and Biscuit bridges-I hear that they're awesome, but I haven't used them yet. Like yourself, I have trouble finding perfect bridge placement/intonation right off the bat-my solution is to use tailpeices on all my builds along with mobile bridges and flat resonators, so that I can move my bridge that critical Millimeter or so...
My choice of resonator ranges from cookie tins, paint can lids, paint can bottoms (waste not want not), candy tins, pot lids...whatever sounds good to me. I don't know if you prefer the 'stick-thru-the-body' method or the 'neck-block' style of instrument but Resonators will work on either one.
If you decide to simply cut a hole in the soundboard and place or glue the Resonator in (good for lids), you might want to brace the Reso with a little ring of wood around the cut, and maybe a little bracing radiating out from there if the soundboard seems fragile.
If you decide to rest the Reso on the back of the box (good for tall Resos like full cookie tins or film cans), you want some small braces or blocks t0 elevate the Reso between 1/2" to 1" off the actual back for maximum vibrations. Also, if there is some space between the Reso and the hole in the lid (vertical or horizontal), you can use that as the actual sound hole if you like. Whichever way you end up mounting, just make sure that you take the final matching of action between nut and bridge into consideration...
I have found that Resonators in a wood or laminate box have a nice, warm tone that softens the ring-if you want a brighter sound I recommend sealing the inside pores with a varnish or shellac, doubly so if you use a Masonite box.