Super Telephoto for Canon

Looking for a cheap telephoto for Canon in the 300mm-400mm range, it's only gonna get used every now and then for some bird shooting, and the photos are mostly for personal use, so it doesn't have to be super sharp either, though I do want something sturdy, since I'm gonna use it a lot while hiking, so nothing that'll get cracked if you look at it the wrong way.
Old, used, and fully manual are all not a problem for me. 

My budget is about $400-$500

Currently I'm looking at the Canon FD 300 F/4 (with EdMika adapter), though not quite sure what's the difference between the L and non L versions of this lens, if anyone can help, or offer alternative choices.

L lenses have weather proofing , and are generally sharper , with better coatings and stronger bodies , but at a price . 
Alternative choices:

1. If you are only going to use it now and then , consider renting a decent telephoto lens when you do need it (They are sometimes up to $10 000-$20 000 if you buy ).


2. Use your cash to buy a cropped sensor body with a 75-300mm lens, effectively giving you  120-480mm focal length.


3.Find a second hand lens on eBay and pray that it is not scratched when it arrives.

AnimeLady wrote:

L lenses have weather proofing , and are generally sharper , with better coatings and stronger bodies , but at a price . 
Alternative choices:

1. If you are only going to use it now and then , consider renting a decent telephoto lens when you do need it (They are sometimes up to $10 000-$20 000 if you buy ).


2. Use your cash to buy a cropped sensor body with a 75-300mm lens, effectively giving you  120-480mm focal length.


3.Find a second hand lens on eBay and pray that it is not scratched when it arrives.

I'm talking about Canon FD, not Canon EOS....even the cheapest one is built like a tank, and non of them was weather sealed btw. They're all used, since they don't manufacture them anymore, not since the 80s, I just want to know the  differences in that particular lens. I really have no problem buying through eBay, I've bought tons of used lenses through it, not a single one had scratches on the glass itself, only minor signs of use, so I'm not worried. 

I don't want a crop camera body and a 75-300, that's probably Canon's worst lens, built with cheap plastic, and a Rebel isn't that sturdy either, plus I don't want to hike with two cameras with me. 

Thanks though.