Goerge,
Your dilemma is not unlike mine on my Nomad and another bike where the seatpost and saddle stand well proud of the seat collar.
I have a plan I want to try when I get back from the tour I will be leaving on tomorrow (a 10-day or so trip from home over the logging roads and singletrack of Oregon's Coast Range on the Nomad, the down the coast about 100km and back on similar routes to home again). I bought one of Thorn/SJS Cycles' 45° T-bars with seatpost shim...
https://www.sjscycles.co.uk/accessories/thorn-saddlebag-bracket-1725-mm-extension-222mm-45-deg/?geoc=US...same without shim here...
https://www.sjscycles.co.uk/accessories/thorn-accessory-bar-t-shaped-1725-mm-extension-45-deg-no-shim/?geoc=US...and have been playing with it for some time to get a mount that will work with my Thudbuster LT seatposts on both bikes.
My dilemma is the Carradice (Camper Longflap in my case) sits just too high when attached to the saddle loops to also sit on the rear racks of these two bikes (does fine on my randonneur bikes with horizontal top tubes and taller frames). It is fine if I place it on a stuff sack filled with rain clothing or my tent but not in their absence. My original thought was to use the 45° T-bar as intended, so I fitted it with allen bolts and acorn nuts as shown in the illustration...but while it cleared the rearward portion of the Thudbuster's down-and-rearward trajectory at the saddle, it still placed the bag too high to be ideal for me.
My next effort will use the 45° T-bar as a back-stop for the Carradice, to give something for the leading side of the bag to rest against, perhaps even secure to and then it won't matter if it is suspended above the rear rack. Lots of creative possibilities there so maybe worth a thought. I considered using some tensioned members (i.e. paracord) to make a soft but inelastic flexible (macrame?) cradle from the T-bar to the saddle rails. The 45° T-bar can even be inverted if desired.
Speaking to another issue you raised, I have lined my Carradice with corroplast salvaged from campaign signs. Midterm elections are coming up soon in the US and the day after those political yard signs are practically worthless. The candidates' office staff are supposed to collect them but usually miss a few. I got mine from overlooked property owners, campaign offices and the recycling center at the local dump --all for free. I have found the paint used to silkscreen the campaign lettering is pretty stable and has not come off or transferred to items carried in my bag. Just a thought and much cheaper than buying a whole large sheet you might never use.
All the best,
Dan.