Could you bend the "L" brackets to fit the curve of the crossbars and add another bolt or screw or two into the crossbar?
This is what I ordered for crossbars, not cheap but not expensive when you consider the cost of prebuilt racks and crossbars:
1" x 2" OD Aluminum Oval
I have also found metal recyclers that sale used stuff. I used one extensively for projects and recycling, check around for one and you will find some inexpensive materials.