WINDOW A/C WITHOUT A WINDOW?

i have a 240 sg feet garage in thE desert gets about 105 in the summer i have a roll up door and a vent about 5 by 12 INCHES near the floor on the back wall I have tried three portable a/c units all three from 9 ooo- 11000 btu blow cold for a few minutes then goes to fan mode i have given up , why they do noT continue to blow COLD ?? anyway can i get a window unit and build a cardboard inclosure so hot air does not go back in the room and have it dump out this vent ? any ideas its HOT.