Glad the storm didn't get out of control for you.
May I suggest you take the time to find a 'mobile' duplexer. Undoubtedly it will make the entire project far more feasible and they are really quite inexpensive.
Says he ships world wide and for $20 will tune it to your specifications. I'd tune it for free for you if you want. I could probably even come up with the cables for you at no charge.
These things are comparatively tiny. That unit has BNC connectors so it looks like it's probably 5" by 7" and about 1" thick.
I know you didn't want to get spendy on this, but that would be just the ticket to make it work and be extremely portable.
If you're into building, I have found some information on how bandpass and notch cavities are built and tuned. Here's a bunch of appropriate links that will raise your knowledge of what you need to acheive. http://www.repeater-builder.com
is a treasure trove of information.http://www.repeater-builder.com/antenna/pdf/beer-barel-cavity.pdfhttp://www.repeater-builder.com/antenna/pdf/ve2azx-duplexerinfo.pdfhttp://www.repeater-builder.com/antenna/pdf/w1gan-duplexer.pdfhttp://www.repeater-builder.com/antenna/pdf/portune-trash-can-cavity-73-july-1996.pdfhttp://www.repeater-builder.com/antenna/pdf/ground-plane-antennas-for-146-220-440-mhz.pdfhttp://www.repeater-builder.com/antenna/pdf/qst-2009-07-coaxial-dipole.pdf
And assuming you have some reading time, here's a great read on many of the topics involved in getting a repeater (or any radio) to work well. Great stuff.http://www.repeater-builder.com/antenna/pdf/andrew-about-rf-communications-a-non-technical-approach.pdf