Didn't you just make a cafe seat? Can't you make a flat piece of metal to fit your fender? Check out McMaster-Carr or someplace and I'm sure you'll find hardware you can use to fix it down.
And if you can't afford to buy a cafe seat but don't think $100 is a lot of money for a flat metal tab, I'm confused. (Edit: Ok, didn't realize it had text embossed for that price, but still...)
If you want one ready-made for cheap, I'd check Ebay and Royal Enfield suppliers from India (bulletwala, royal enfield zone, etc.) Those license plates are popular over here and parts are generally next to free by rest-of-the-world standards. (Although for selling out of the country, they tack on a relatively huge upcharge.)