Not really MYOG, more like 'Assemble your own gear'.

I used Rixen and Kaul Klickfix parts from Amazon to make a modular seat post mounted drybag holder.







KF850 bracket http://www.amazon.co.uk/gp/product/B000OZDKGW
Seat post extender http://www.amazon.co.uk/dp/B001VS7UJY
Spare fixing plate kit http://www.amazon.co.uk/gp/product/B001CCNCUU

Total cost for the Klickfix bits: £23 (koala is £65)
Weight : 440g (koala is 230g)

The foam is cut from military rollmat and the strap is a spare Alpkit one.
The strap and foam are held captive between the two ABS plastic plates that come in the plate kit.

Pretty solid with a couple of kilos in it.
No leg clearance issues so far. I'll have to see if I can get far back enough for very steep downhills though.
Spare plates are only six quid so you can bodge whatever you like on there (up to 5kg).

No WRT for me I'm afraid. SDW next week though!

Good effort - with some adjustments it looks like that could be made dropper seatpost compatible?

My only concern would be the durability of the foam around the fixing points to the mounting plate.

I did think about that; the foam is extremely tough though. Backing it with thin flexible plastic (think ring binder cover) would spread the load around the mounting plate.
As for a dropper seat post, it does need mounting as high as possible or it fouls the rear wheel. That probably means hefty shimming to meet the narrower diameter.