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Going adventure biking next week and I want a large frame bag to fit the whole triangle, can anyone recommend a decent one that fits loads in?
With that short a warning you aren't going to get a custom bag made. Check Apidura https://www.apidura.com/shop/ who have a sizing guide which indicates if one of their bags will fit your frame. I was also going to suggest Alpamayo but their site indicates that they are out of stock of their full frame bags. Revelate also do standard bags, check either Backcountry.scot or CTBM
Another option might be to contact Rich and Shona at Keep Pedalling in Manchester since they stock quite a bit of bikepacking kit. Another shop who I know have bags are Ghyllside Cycles in Ambleside. There's also the Alpkit shops.
Whoops, realised you wanted a frame bag. Sorry.