Custom bar bag, thoughts?

I've been having a look for a bar bag to use for my commute that I can also use during the day with minimal fuss. I'm not too keen on the Carradice style box-bags and am currently using the Restrap bar bag. (https://restrap.co.uk/products/carryeverything-bar-bag-holster)

The Restrap one is good, but not too fab for carrying around in the day/having to take the holster off the bike every time it's locked up. So, what I'm looking for is something that looks fairly nice but can attach to a front Randonneur rack quickly. This is the rack I've got on at the moment:

I've always really liked the look of the Big x Top duffel and I've used the daysack for ages and loved it: https://www.bigxtop.com/collections/bags/products/duffel, so was thinking of getting a custom one made that could attach to the rack fairly easily.

Here are my theoretical modifications to the bag:

1. A small loop of fabric at the rear of the bag to fit over the vertical section of the rack, closest to the frame. This would act as a fairly rudimentary decaleur
   
2. Two velcro, or clasp straps along the bottom of the bag. This should allow me to easily strap it to the base of the rack and combined with the small loop, make it secure.
   
3. Reduce the width of the bag to 40cm so it can fit between my bars.
   
4. Attach a semi-rigid plastic panel to the base of the bag so it doesn't droop over the fairly narrow rack.
   
   

My question is whether people think this would work? I've run it over in my head, but I figured I'd see if anyone who's got more experience with bar-bags/making them could see any glaring issues with the plan before I go ahead and see if Big X Top can make it.