Good question, I'm not deep in The Black Series, but bought a couple Rexies of the first run when
StarActionFigures had them on sale for 10 GPB. The second run sold right through for 20, but the new one with Poncho is still up for pre-order there.
Have you looked in these various Facebook groups, yet? Couldn't find any specific idea on YouTube in my 2 minute research.
CenturionsCorner or
Ka-bashed Figures might be helpful to inspire.
Personally, I would buy a bootleg Clone with the new body from ebay or AliExpress. They're about $16. Then, I'd get a resin cast or 3D print of Rex' helmet, pauldron and holster-belt. There are so many great sources out there. Star Wars: A Custom 1:12 Galaxy is just one of them.
Second option is to buy a new Clone Body e.g. the Kamino Clone or 212th Clone Trooper (are there any more at the moment?) and the new BB Rex and combine both. But that's probably the option you already came up with. Hope my comment was at least a little helpful...
Yeah, if you get the first version of Rex anywhere cheap, I'd go with this one.
The Kamino Clone is probably the cheapest to go with when it comes to availability. Best regards & good luck with your project!