will cz75 extended baseplates fit in idpa box?

I kind of doubt it as my Shadow is not a loose fit.
But why do you care?
You can only load 10 +1 in IDPA anyway. Use the OEM baseplate.

Diesel McBadass said:

One, i only own 4 mags, 2 16 and 2 18, and dont want to have to buy more expensive mags. Second, only 10 rounds? I thought there was a limited 10 competition for that?

Click to expand...

You said 'Extended Base' mags.
An 18 round mag with the plastic base that is a little less than 1/2" thick, on my Shadow, will fit the box. The magazine extends a little less than 1/2" from the base of the pistol. This is not an extended base just a plastic as opposed to a steel shoe base.

There is NO class of IDPA where you can load more than 10 rounds plus one in the chamber. A numkber with less.
Limited 10 is a USPSA division, not IDPA. USPSA doesn't have a box.
http://www.lcsa.info/index.php?option=com_content&view=article&id=12&Itemid=101

Plus in a hurry between stages it's way, way easier to load 10 rounds in an 18 round mag than 10 in a 10 round mag. That last round or 2 in a 10 rounder can be tight.