Paul,

No matter what you do now, you either have to accept a "hammer-fit" as you put it; Or, place sufficient packing between tabs and seat-frame spine to allow an easier fit that can then be clamped tight.
Paint doesn't really take up much room (depth) so you don't need much clearance.

The clamp-plates need to be 3mm thick (minimum) or the ends where the bolts are just bend inwards where you apply the clamping force without actually gripping the spine of the bike (don't ask how I know).

If you have any really thin steel (0.5mm) use that or even some cut-up bean can (its function is just as a spacer). All you are looking to do is introduce some clearance.
If you use some really thin steel when you clamp it tight you can test for "snug-fit". Then weld the edges and add filler material. That's going to be immovable I think (dodgy though it might sound).

The trouble with making the gap far wider and then "lining" it is that the linings tend themselves to be a bit compressible and you can end up with less of a tight fit than you would like.
The old WD seat clamp was lined with leather from the upper of an old pair of shoes and it served me well.

Otherwise, use 4mm plate for the tabs and just file/flap disk a minimum amount off the insides so its a snug but non-paint-scraping fit. Same result, different method.