Hi, I built a 1x15 and a 1x12 a few years ago from Watkins (WEM) plans. These were linked with Celestion suggestions. I took it along to Charlie Watkins who tested it for me and gave me tea and bikkies. (Great guy with lots of stories). The 1x12 was just a vented box for which I changed the dimensions but kept the same total volume. I matched the dimensions so that it would sit under my 1x10 combo and then recovered the combo in matching material. The 1x15 dimensions I left as designed. Taller and thinner than most 1x15's but it carries through doorways without skinning knuckles or strange twists.
The beauty about building your own is the ability to use more sensitive drivers. More sound output fot the same input. Always check the sensitivity rating of a speaker. The power rating alone only tells you what it takes in, not what it puts out. Sensitivity is measured in dB/W/m and an increase of 3dB is a doubling of power. My 103dB/W/m 15in speaker at 100W puts out the same sound as a 97dB/W/m speaker at 400W! As with many things however the sensitivity can be measured differently so not all drivers can be compared directly.
I did have lots of pics and an article about how to build it which I supplied to Bassist Mag for them to run. They kept it for a year, decided not to run it as it might upset their advertising income from speaker manufacturers and then lost the article and pics. Happy to offer any help I can.
Graham