The terminal cup shouldn't be more than $5. Parts-Express.com has a bunch
Search Results Page
I've even just drilled two holes and put bolts through them. And I've drilled a single hole and siliconed a wire in it. You don't need something fancy. You're mounting the amp to the box. It only has to stick out a foot or less to reach the terminal of the amp. Don't make it too difficult. Beside, the smaller the hole, the better.