Finally got hold of the photos of the rack, after getting hold my father's digicam. As you can see the Topeak Drybag Panniers are compact and small when it has no items inside. It can be rolled down to this level.
If the need for extra cargo and haul is needed... flipping the top fold open, and you've got this. As Topeak stated it can hold 42 liters of cargo. Though with 42 Liters, imagine the weight of your whole rig. At full load my bike weighed at a total of 61.22lbs. Truly a heavyweight indeed.
The Bags come supplied with spacers so just in case you need that extra tight clamping. Although I still don't find it secure with these spacers, that I often use electric tape to wrap it around the rack's rails and the bag clamps.
And finally the Max 10kg seatpost-mounted carrier I bought at Paulinas for 950php.
The rack can carry a total of 10kg, and of course can be mounted on any seatpost with its quick release lock.
Sadly with only 10kgs. maximum weight limit, carrying 42L is impossible.
Oh! I won't be using the Diamond Back for the epic trip, as I'll be using the Vision Pursuit. Riding a Single-Speed to Baguio?! That would be insane.