I would stick with your flap-jacks.
The 75g ones I get from (such as) B&M cost 19p, they weigh 75 grams, have typically 333Kcals, 4 grams of saturated fat and 13.5 grams of sugar.
Being a 'recovered' diabetic (no longer on medication) I do have to take an interest in what I'm eating and find that I am 'walking off' more calories than I am eating. The flap-jacks pose no problem at all (and they taste good)
My typical daily intake is 2000 Kcals and my outgoings approximately 2200-2300 Kcals
Typical days meal Plan :
A number of cups of tea (with sweeteners) during the day.
Breakfast = Porridge
Lunch = Cup -A-Soup &/or 2x Granola bars, or Flapjack (depends on if in the mood to set up the stove)
Evening Meal = De-hydrated meal (typically 800+ Kcals)
Supper = Mug cocoa, 40g bar of chocolate, Flapjack.
There will probably be another flapjack consumed as needed.
My rations are split up into day-packs or 2-day packs and sealed in a foil 'zip-lok bag'. There will generally be an extra bar of choc or flapjack 'above' the planned menu.
I cannot think of any savoury product (apart from nuts) that will survive in the bottom of a pack for a couple of weeks.
Mixed dried fruit ( or even just sultanas, or raisins), are high in carbs & are a good snack-boost for "11's" or "3's"