Good idea on the steel tubing. Use some small diameter, like fuel or brake line, that fits the spring closely. If the break is in the middle of the coil, you caould bend the tubing to the same diameter as the coil and long enough slip over about 2 inches of each broken spring tip to resist twisting as well as bending forces.

Another suggestion (if my used mattress idea is too weird for some) go to an automobile salvage yard and examine seats in several different cars until you find some springs similar to your broken one. Find a suitable car seat that has ruined upholstery and/or a broken frame and hopefully you gan get it cheap.

Happy Motoring, Mark

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DrDKW