It's a little ugly, but I've had good luck with stacking cardboard half gallon milk containers (cut the bottom off of one and stick it in the other). You get a tall container that avocados do great in, and when you plant you don't have to disturb the roots at all -- just put the whole thing in the planting hole, cut the bottom, slide it out from underneath, fill around with soil, and then slip the whole double-height container off the top.
Edited to add: mango seeds sometimes do well in these, and sometimes not. Really big seeds don't fit, but smaller ones do well.