This is a quick and inexpensive (~C$35) method to add a 2.5 litre framebag to the bottom of the frame triangle. MEC sells a suitable framebag that will fit most frames without any modification beyond cutting off mounting straps. The dimensions of this particular bag are sized to match reasonably well with the typical bottle-boss points on many frames. The perimeter stiffener of this bag results in an installation with excellent stability and minimal deformation.
The general sequence is:
1.) remove cages and screws from frame
2.) test fit bag to frame before removing any straps, and determine if any straps should not be removed
3.) remove straps from the bag and heatseal cut ends
4.) position and centre bag in the frame and mark location of bosses on the inside of the framebag
5.) punch holes in the bag
6.) insert screws into holes and mount bag to bosses.
The resulting installation will hold a couple of 1 litre soft flasks or a flask and a bike bottle. The remaining space can hold a spare tube and pump, and multitool, or other small items.