roll bag questions

Been checking out dowco and saddleman bags. 1) Has anyone found a cheaper effective alternative 2) is 18 by 10"" ( 47 by 25 cm.) the way to go 3) an easy, secure way to tie it down on passenger seat.
Thx

I used to have this on and i used it on my fzs600 and my sym gts300. Until i had to take a passenger with me. http://www.petes-superbike.com/cylinder-tail-silver-range-33ltr-p-46225.html

And these ar some of the cheaper options:
givi:
https://www.rad.eu/nl/c/p/32164-givi-waterdichte-rollbag-wp402.htm
held:
https://www.rad.eu/nl/c/p/39873-held-roll-bag-4332-00-014-40.htm

I consider bag/bags being waterproof completely necessary---Ortlieb work for me(I have 2)service life more than ten years----simple cord (2) keep it them on passenger seat.

You may want to consider this bag since it is also a backrest.

http://www.bikersfriend.com/

I have found that this combo works well for me.  A Dry bag 40L+ bungied to the pillion seat.

Dry Bag $15 and up.

Bungee Net Typically $5-7

purcased a givi roll bag-130 cdn and a bungee net. This way I can access bag without a lot of hoopla.

I use my 50 year old Army duffel bag. Tent, sleeping bag stuffed in; great back-rest. Harley

I've been continuously using a couple of different sized red Ortlieb rack pack bags I bought from BMW specialists when they were based in the spa town of Bad Neuenahr-Ahrweiler back in the Nineties. They're tough, waterproof, and riding solo I strap them behind me and use them as a back rest. I don't bungee them on as I can cinch them on by threading the Fastex buckles/straps on both sides of the bag through the grab rails on the Silver Wing and back onto the bag. I cover the bags with a Fastex steel mesh cargo net and lock them onto the seat if I'm off wandering about somewhere.

In with all the other junk I've collected are a pair of new roll top dry bags similar to Ortlieb fitted with straps to secure them on top of a pair of hard panniers/saddlebags. They're made by Wüdo another German  BMW specialist, I've yet to think of how I can use/convert them for maxi scooter use.  

I have a Bikers Friend hard roll-bag as in the link Jeff has posted.
All in all it's very good, the only downside being that it's sealed/air-tight when closed so that if I use it empty, just as a backrest, condensation has been known to form inside.

I use this tail bag on my GT. It fits nicely on the back of the seat in front of my top box. Waterproof and very well made. Pretty pricey at $250 but definitely first cabin. It's semi-rigid and makes I nice backrest as well.

http://www.webbikeworld.com/motorcycle-luggage/bags-connection-evo-rear-bag-review/

http://www.revzilla.com/motorcycle/sw-motech-evo-rearbag-tail-bag