Looking at just the bin and setup files i have around 9.5gb.
Those are too large to even burn as data files to a dual layer disc for safe keeping.

All i can think to do now is to burn most of the install files to one DL disc and the rest + the bonus extras to a second disc for safe keeping.
But then i'd have to copy all the files back to a folder on my HDD every time i wanna re-install.

Anyone manage to burn a backup copy of the install files to a DL disc and care to share how you did it?
I mean there has to be a way, since the retail version obviously comes on discs.

Once the downloader has finished, no one file is larger than 2GB. Individual files can then be burned to DVD as necessary, and you don't need to use dual-layer DVDs.

From my personal experience as a person that works on IT and Software Development and someone that used to Burn CDs and DVDs like crazy I consider it a waste of time and unsafe method for backup.
Pros:
Single disc. Single Item. Whatever may be in that disks if you're organized enough should be fine.
Cons: An Optical disk is small and optical media is SLOOOOOOWWW. Not to mention that is far more damageable than magnetic medi.

Just get a Thermaltake Blackx external hard drive platform and pop a 40 bucks 1 TB hard disk in it.
It will ocupy less physical space and backups are way faster and easier.
For All your files always keep at least one backup of your backup.

Can't you just re-download it from GOG?

If I am not mistaken use two disks and install from them. Escape out of the file check and you should be able to install - change discs when it asks for the missing file. I have not done this yet but plan to soon.

buy an internal HDD and a cheap case to turn it into an external HDD.
much cheaper than just buying an External HDD and would be cheaper than buying DVD-R (for the same amount of storage)

This^^^ I bought 2x 2TB external HDDs and back up my stuff to both in case one fails. My burner only gets used for making music CDs from MP3s now.

The Thermaltake BlackX does that just fine. You can pop in and swap drives with One Button.

I burned the setup files and the CE bonus files onto one dual layer and one normal dvd+r.

Then I grabbed a cover designer software and build me a custom made dvd cover out of the retail cover and a neat backside from a CE bonus artwork, plus a gog.com writing. ^^