There is several ways to mount but the easiest thing to do First is to show us a picture of the boat where you want to mount it..

Just post picture of inside your boat and people here will help you out

Cheers

Randy

hi the boat is an 57" apache with 5" rails in it

When I know I can put the box in a boat and it won't be in the way for maintenance (which is easy to do in a big Apache) I like a "no holes" method. Something like 3m 5200 sealant is hard to beat for a permanent installation.


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i was hoping to secure it with screws / bolts for easy removing

I would Use couple through each side. Maybe 8-32 or 4mm. Use locknuts or blue loctite on so they don't come loose.
Screws tend to vibrate out

I make all miine removable. If there is a gap between box and rails add wood shim to each side of box.

Make sure you don't have pressure on sides from bolts or box may crack.

I sometimes use nylon 8-32 bolts , nuts from Home Depot

Randy

I mount mine the same way as Randy does except I use blind nuts tacked into place with a drop of CA glue. Somewhere I got some rubber backed #8 washers to help seal around the hole. They may have come from Micro Fasteners.
Found them. www.microfasteners.com/product...id=17859&page=1