@ Buje: It will be difficult to place an 18" driver on a 17" baffle (it can be done though). I usually make a 2 part plywood ring (two rings of at least 18 mm thickness), with t-nuts embedded within it. The first ring fits the 15" bolt pattern and is large enough to fit the 18" driver, the second ring allows the 18" to make at least 18 mm of excursion (at least Xmax but preferably Xmech). The rings are glued together but the structure as a whole is removable, so you can still mount 15" drivers later.
Alternatively you can glue an 18" + diameter plywood spacer ring to the baffle directly (with t-nuts embedded within it), front mount the 18" driver on the spacer ring.
Greetings from Zimbabwe. I want to build a pair of Cubo 15 standard version and load them with p. audio blue monster bm-15w 400w speakers. I would like to find out if those speakers are suitable for those cabinets.Plizz assist
The T/S-parameters you send me are of the BM-15LF, whereas you mention the BM-15W here. As far as I can tell those are two different drivers, the BM-15W has substantial lower Xmax, Qts and power handling, however both are best suited for Cubo 15 Extended.