Need rack mounting extenders
 

The extenders that came with my rear rack are too short to allow the rack to be mounted level. They have an 8" working length. I'll need 10 or 12" extenders.
Several sources sell the long extenders, but they are expensive.
What material would you use to make them? They are approximately 5/8" wide by .055" thick, give or take. The material is harder and more springy than, say, galvanized sheet metal.
I have access to some metal working equipment, and I would like to avoid shelling out $10-$12 for some strips of steel if I don't have to.
Thanks in advance.

It's stainless steel. Any shop that does fender installation should have stays lying around. Also, the rack doesn't have to be mounted to the braze-ons. They can go to the seatpost or the collar (special collars with eyelets), or the frame itself (p-clamps etc). The opportunity cost alone would cost more than the $10 for those stays, stop being a cheapskate.

Unless you get heel-clearance issues, there is no reason - except maybe aesthetic - that the rack has to be perfectly level.
Even stays (I am pretty sure this is what you mean by 'extenders') that do not meet your exact needs can bring the rack closer to level without too much difficulty. Pre-bent stays can be nicely flattened out if squeezed in a bench vise, and straight stays can be bent to fit. And short stays can be bolted together to make one longer stay.

I've fabricated several sets using 1/2" aluminum channel that's available at Home Depot or the hardware store. It's a little labor intensive but, when finished, they actually look better than the strips that come with the racks.

Delta makes long rack stays for the Megarack for $10 (look about halfway down the page). They should work for most racks. I've used them on my wife's bikes because her frame is so tiny that normal racks stays are way too short.

I'll probably have to go this route. But first I had to see if there was a cheap, readily available alternative.

I usually just go the the LBS when I have some that are too short. They usually have a box full of that kind of stuff and give it to me free or a couple of bucks. You can bolt a couple together to get the length you need. A vise and adjustable wrench/hammer to put needed bends in. You can even bend a 90 over a counter edge depending on the way you need the 90. might need to drill ahole or 2 if prepunched ones don't work out.

Your LBS should have a whole bag full of them.

I've gotten free or really cheap ones @ my LBS.

That being said for extreme cases, like my 5'2 roommates bicycle, the wrench @ the LBS fabricated some extenders in about 30 minutes. Bits of what seem to be half inch-ish flats of either aluminum or stainless, ground smooth with two holes drilled in either end. Made it perfectly level.

I went to the LBS Saturday and they gave me 2 new short ones. They had a whole bin full.
The longer ones are harder to come by, however. I will braze or bolt them together to make longer ones.

Sure beats paying $10 +shipping off the internet.