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Howdy folks.
After playing around with carrying more on the bike in various locations - frame, handlebars, seatpost - I prefer the framebag.
Over the years I have accumulated a few bags of varying sizes but they are all dwarfed by the gargantuan void that is the space in the middle fo my Genesis Longitude!

I'd like a custom pack that retains at least the ST mounted cage position but ideally both but makes full use of the remaining space.
So who to go to? I have the option (I guess) of using the three threaded bosses on the underside of the TT that aren't currently being used for a hard mount?????
Failing custom recommendations for mahoosive off peg bags would be welcomed 🙂
Don't Alpkit/Sonder do this? perhaps Restrap?
Aiguille, Alpkit, Wildcat and Restrap would all my on my list.
Don't know if Wildcat are still doing custom, though.
Straight Cut are another https://www.straightcutdesign.com/collections/bags
I went with Restrap for mine, the process was really straightforward in drawing up the template and marking up where strap, inner divider etc went. They have a YouTube video on it too.
The only thing I probably should have done is maybe add a few mm on around the edge as it’s fine when not completely full but can be a bit stretched when completely stuffed.
Here it is on the Calder Divide last year.
I know there are other, smaller, makers who will craft one, but I got a an Alpkit one a few years ago. Did the template thing (measure tw... thrice! before sending off) and I'm very happy with it. Fits my long frame well, not sure about my colour choices though!
(will try to upload photo but it's telling me only full members can do so. bloody site!)
Following this as I want a partial frame bag for my Topstone and I was already considering Restrap.
Beerbabe.co.uk if you want bespoke upcycled bags, practical and feel good about yourself too
Not used their custom option but Wizard Works are worth a look for good quality, London made bags.
Straightcut Design! Their bags are top quality, both materials & workmanship. Easy ordering & custom sizing.
Bikepacking friend sings the praises of Camelchops here in Scotland

Success!
I've had two custom framebags from Alpkit: a small one for my Tallboy and a full-sized one for my old Cotic Solaris. I got lucky with the second one as it also fits my Mason perfectly. Very happy with them, although I'd suggest taking a few mm off the overall size (ie. trim the template down by 5mm).
Used Restrap recently for a frame bag. Very pleased with it. Great customer service too.
I've used these folk. Very good and robust bag and a good price
https://www.buggybags.co.uk/pages/Bike%20Bag/Design%20Your%20Own%20Bag.htm
Beerbabe.co.uk fan here too, Judy is awesome talented and very helpful at a price that very reasonable.
Me and my other half both have Camel Chops bags, fancy colours (or plain), very helpful and will make things exactly as you wish.
I’ve got standard restrap bags and I’m happy with them, they feel a better product than our original Alpkit bags. @ahsat has a custom frame bag from them, the process worked well.
Ross at Straight Cut Design - lovely fella, top quality kit 👌
@scotroutes - I've got a 7yr old large Restrap frame bag that has stood up to a fair bit of abuse. It's frequently bulging with too much stuff but just works really well
I'd replace it immediately if I lost it
Another shout for Strsight Cut.
Based in Edinburgh. Small producer. Top quality.
Camelchops would be on my list, as much for the ace colours as much as they seem really well made.
https://www.camelchopsgear.co.uk/
I’ve got a custom fat Alpkit frame bag for my Longitude. Just rode 2500km across Italy, the Alps and then France with it, no problems. Well made, good value. I liked that I could specify the angle of the lower zip to give a horizontal floor to the upper compartment.

I was tossing up between Restrap and Straight Cut Design here, and went with Restrap just because the other bags I have are Restrap and I like things to be matchy matchy. Dead straightforward as others have said.

Another for Camelchops here. Good communication and very well made bags.
I've had three now from Alpkit, which have all seen plenty use. No issues to report.
With a big frame, make sure you get a zip / compartment to split it up regardless of where you get it made. You'll end up with a giant bulging mess otherwise.