I don't mountain bike but I have tried lots of products to mount gopros on my bicycle handlebars, most of which don't work, sometimes for reasons that are probably not relevant to you. For tightening I've used the little wrench things you can buy, but it's a nuisance. I've found these bolts to be hand-tightenable enough for most situations: https://www.amazon.com/gp/product/B09L5W1V47/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

There are situations where the extra large handle gets in the way, so it's not a magic solution for all situations.