Those marine units start around $1,200.

You could also get an RV roof unit but they're also quite large (overkill) and more expensive than needed.

We buy some 2,500 BTU air conditioning units for electrical cabinets - I've always thought about building a shroud for one of those and mounting it through the roof of the cap (they're long and slender too). But they start around $800 as well.

Then again, an ice cooler, a small automotive evaporator core, a water pump, and a small fan and you can create your own ice cooler. Our pilot built one for our company planes and it will cool down the inside of that plane in a few minutes...I don't think you could expect it to last more than 3 or 4 hours even with a light load though.