The dipole peak is caused by the interaction between the output from the front of the speaker and the back of the speaker. Depending on the width of the baffle it will cancel or reinforce at different frequencies, so you get a pronounced peak in the response where it reinforces (adds), and dips in the response where it cancels. This is most obvious in the first dipole peak at the lowest range of the driver's response, but also has an effect on the frequencies above that.
To eliminate it altogether you need to make the baffle wider (at the top and bottom as well) than the lowest frequency you want to get from the driver. That's typically wider than practical.
Another way to deal with it is to use separate drivers and vary the width of the baffle where they are located to get the best response. I did that in my main speaker build where I used a 15" woofer for the lowest range on a baffle with deep wings, then used progressively smaller drivers on the baffle as it narrows.
The dipole peak problem was responsible for some of difficulty I had, but it was also a hump in the tweeter's response added to it that was the major cause. By shifting the crossover frequency up to 4KHz, it took a lot of the thunder out of the hump and flattened the response to an acceptable smoothness.
The 5" midwoofer is happy to play up that high (as confirmed by the distortion measurements) and whenever possible I prefer to cross the tweeter higher to take away the stain of using it at lower frequencies. This may affect the off-axis response of the speaker, but the way I'm using these speakers it would be better if they were more directional.

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