I don't know enough about IIR filters to actually make one. .
You can get some theoretic explanations on JOS homepage:
https://ccrma.stanford.edu/~jos/pasp/Phasing_2nd_Order_Allpass_Filters.html
There are some published allpass filters in this thread:
Here is a small phaser example based on the tiar/filter/APChain object.
testPhaser.axp (6.6 KB)