As an alternative to buying a normal style faceplate casting and adapting it to fit a backplate consider making your own plate in "grid of tapped holes" style and fixing it to whatever backplate you manage to obtain.
As regular forumites will know I'm no great fan of T slot and clamp work piece mounts for our sizes of work as they are all too often cumbersome and difficult to install. Quite modest clamp forces are sufficient for small jobs. Especially as you can often fit more, small, lighter clamps in.
Some years ago I made a grid of holes faceplate in that style for a man with a Smart & Brown model L lathe. I sliced about 3/4" off a length 7" diameter alloy bar that I happened to have in the workshop. Faced both sides and applied a 5 spot dice pattern of M6 holes on 1" or 25 mm centres. I made a plain bore, possibly around 1" diameter in the centre which he finish bored true on his machine. Very useful when shifting work held in fixtures between machines. The finished plate was bolted to a small chuck backplate that I re-machined to fit the Smart & Brown spindle. I was subsequently told that it worked very well indeed for his work.
One potential issue with this style of alloy plate is marking where clamp stand-offs and the like sit. There are various ways of carrying the load. I suspect on small jobs screwing a stud into one of the other holes as a fulcrum for a simple bar clamp would work as well as any. The plate is easily skimmed if it does get damaged.
If you expect to be doing a lot of face plate work on tricky stuff consider making a second, plain, front plate for sacrificial use. Bolt it on and machine pockets, grooves threaded holes in odd places or whatever you need to hold that impossiblium job in the right place.
Model engineers as a breed tend to be excessively chary of sacrificial, machined for the job, holders. I'm as bad as any but find that when I do weaken the actual cost of consumed material is small and time saved great.
Clive