Matt,
I have great respect for all Aruba's RF gurus out there. I had the privilege to had Eric Johnson locked in a conference room with us twice, where we openly discussed of various deployment problems and solutions to resolve them. Let me tell you that he had solutions that we could not believe of. To make matter worst, all of them worked perfectly.
I had also the opportunity to get my hands dirty on some nasty stuff. (You know , the kind of stuff where up to eight radios can be crammed together in a single housing...) Anyway, there is some truth about having radios too close in the same band as they do backfeed each other, thus increasing the noise floor. Of course, if you are careful when choosing the channels in the 5GHz band each will use, you can mitigate the issue but you won't be able to eliminate it entirely. The problem worsen in the 2.4GHz band where there is no mitigation possible, as the three channels barely clear each other. There is no workaround for this problem in that later band. You can quickly setup a lab with three AP and a controller to clear this out. Set two AP as access points, have some data transfer ongoing to a couple stations and have the third AP in spectrum analysis.
None of us were aware of your purchasing deal and your university's location specificities so I won't challenge your choice to use the AP300 series. In Canuckistan, there is really no other options than installing outdoor rated AP, unless you hide the indoor AP inside while having the external antenna mounted outside.
If you really want to go cheap for this temporary setup, you can use an inverted bucket, small trash can or a caped short section of plumbing pipe to cover your AP. Rule of thumb is to have that container twice as long as the vertical measurement of the AP that it will protect. Unless the container's material crack or is broken, water won't get in it.
Another option to get the job done will be to use a lenght of metal pipe as mast. Whatever vendors tells you, it does not need to be collapsible: it needs to be safe. It needs to get a good, sturdy and properly weighted base, which can be compensated by adding guy-wires. I'll let you and/or fellow engineers and mathematical gurus at the university figuring this out.
Lastly, gives a chance to the good old TV coax cable when you'll be thinking of a cheap solution for this project backhaul needs. RG6 cable is far cheaper than any twisted pair contraptions and can provide you 1000BaseT at 2.4km. You can even reuse a coax cable in service if you put the proper RF filters on. It might be just plain luck but I had no adapters failing on any installations I've done since 2009. You'll find plenty of information about this if you search "ethernet over coax" on any good search engine.
Keep us posted with your successes and failures.
(BTW, I'm still open for a call if need be.)