I'd say if you're not in the position to buy a new one, make a frankenkeezer
. Mount it higher as cold air is more dense and will sink once inside (or don't and just make sure you've got air movement inside). If it was me, I'd mount it so that the lid of the freezer sits right on the top of the AC (less cuts, easier to install, easier to insulate). This will also allow you to use a larger bucket to collect condensation water. Figure out what you're going to build the AC support from and keep it simple.
If it starts to feel like it's not worth it while you're planning it, ditch it. Either hit up craigslist for a lightly used one or pick up a new one. They're really not too expensive and I'm not sure how old your other unit was but a newer one surely costs less to run.