Making bi-polars out of bookshelf speakers ?

Well, you can certainly do it. But manufacturing speakers is more than just building a box and putting some drivers and a crossover in it. Good speaker designers are going to be doing a lot of testing with structure and components, both measuring and by ear, in order to get it right.

One issue for example...if the speakers are ported, the port may be tuned to augment the response of the speaker's bass driver. The size of the port, the volume of the speaker, the characteristics of the driver will all interact in the tuning of the port. If you take a bookshelf and alter its cabinet, it may sound completely different unless you are altering other components as well.

I think it would be a fun thing to do, but unless you know something about speaker design, you should regard it as an experiment while you're learning. I think that the humdreds of dollars you save will quickly disappear as you revise your speaker design. You should view this as a hobby which is its own reward rather than a cost saving measure, unless you are building from a spec that somebody has already done. I would certainly recommend that you seek out the DIY forums for more comprehensive information than you might get on a general Audiogon thread.

Thanks Markphd ... I should have added a little more information. I've built MANY speakers over many years, and understand all of the variables involved... I've just never built a bi-polar speaker before and was wondering if there was anything I may not have considered. I'll keep the correct enclosure volume, as well as proper port length and diameter. I just wanted to be sure I was not overlooking anything else.

While you're at it, you may want to consider installing a switch to swap phase on one set of drivers. This will give you a switchable bi/di polar array.
My maggies have a 'dead spot' when moving the ear across the edge of the panel, the front/back wave cancel and the result is a null, since the front/back wave is out of phase.
Some of the multi-channel protocols called for this type of array in side/rear channels. An all-Magnepan HT setup should be pretty boggling....if you have the space! I sure don't!