Good luck with the repair...just a few things to consider...

The x-2 FCS rail plugs need to be attached to the deck of the board in order to spare you from any issues in the future. Most rail fin failures are either caused by and incredible amount of stress or by poor installation (which it sounds like the latter in this case).

The FCS install kit comes with a number of specialty tools outside of the "holesaw." These allow you to not damage the integrity of the foam around the center hub of the foam attached to the deck of the board. Essentially this helps with the integrity of the install. Go online and find a picture of the kit itself...look at the tools needed...if you think you can rig something do it!

Marine epoxy will work, but if it were me I would just purchase regular poly--also, it may just be worth it to buy a finbox repair kit (emergency) and some resin. You get the plugs, filler, etc. but then I would use the poly to set it. As for the filler...its q-cell...not provide any real significant strenth--but it does look pretty...you could add cut fibers will make it stronger, but not worth it as you will likely get a lot of air bubbles.

To be honest, I understand that you want to attempt this yourself, but I really don think it will be worth it in the long run...not worth the time and money necessary to get a decent fix. Realize that you blew out both plugs and there is likely quite a bit of damage to the foam below the boxes...further, with damage like this I have typically seen the glass delam'd around the boxes due to the force applied to the fin. Just pulls up the glass...Consequently, your repair is going to become a bit more extensive...at that point...you'll need glass, orbital/palm sander and sanding down the box itself is tricky because the likelihood of over sanding is high, etc. Realistically, this is going to take you a bit of time...more than you think it will...a shop could probably do it in 3 days and spare you a ton of frustration...

However, if your anything like me you're going to ignore the point above...if so, check swaylocks and search for homemade FCS installs...I've seen threads over there where people make their own jigs, tools, etc. It will help you incredibly...further, watch the installation video (http://www.foamez.com/page_2.html). THis will provide you with some perspective...lastly, if you need to talk to someone while in process...call Greenlight...Brian posted above and has done a ton of these installs...he could provide you some insight into trouble areas you may find...

Well...hope it works out...if it comes out to your satisfaction post a thread about the process...I';m sure there are plenty of people on here interested....