Another possibility for those aluminum stays are rulers, be they exactly 24 inches, 24+ in some styles, or 36" and cut down to 25". Could be as little as $3-6 each at Home Depot, Walmart, or off of eBay out of Shanghai.
Or, to be old-school about it - the ORIGINAL composite material - wood. A wooden yardstick at Homer Depot is $0.78.
Cut to length and varnish it to reduce water absorption. If you want it to be even stronger and/or stiffer, put some fiberglass on each side. A super-cheap way to do that (while not as strong as, say 6-ounce FG clothe two-part-amine-epoxied onto the wood) is to apply adhesive fiberglass joint tape sold for use on drywall or backboard:
Use varnish or paint as your combo sealant / affixant. Heck, buy 4 yardsticks, FG and paint three of them. Then stress to destruction a plain one and an FG-ed one.
Aluminum is, of course, potentially bendable to conform to your back. But if straight stays suffice, going with wood will certainly give you more stiffness with less weight than aluminum.