I skipped the brackets entirely for some 1/4" SS bolts and SS fender washers on my MJ trailer. other guys have done as you are thinking and just use the brackets as giant washers. still other guys weld bolts to the brackets. it doesn't matter much as long as the fenders stay put at freeway speeds
I run an 8" sub and it'll liquefy your kidneys if it's behind the driver's seat.
it's a tiny cab. you don't have to get too crazy and eat up all your storage space.
price is decent, but man that shipping to the US is crazy. I imagine the canadian members would get it cheaper.
https://www.ebay.com/itm/125147375414?hash=item1d235d6736:g:xnsAAOSwuDxiCWOM