A room 9 x 9 x 7 blocks high. I like to make mine out of glass so I can watch the blazes. (Edit to note that it must be 7 blocks high, starting with the spawner itself. Leave two blocks above the spawner so you have 9 blocks high in total. The pictures shows one with 3 blocks above the spawner open, but only two are needed.)
In one corner at the bottom, an opening big enough for blazes to go through, but blocked by open trap doors. The blazes think they can get through so they gather in that corner trying to get out.
Under that corner, a hole with a trap door. The blazes fall in, drop three blocks to a lower half slab where I stand and smash them. They can't see me, but I can hack at their feet.
I sometimes add a piston with a comparator timer to damage them making killing easier.
0 to 30 XP in about 10 minutes and fast, fast mending on tools, weapons and armor that have the mending enchantment.
Edit again to note that I cover the floor in slabs to prevent pigmen from spawning.