Nick, there are some rough faceplate dimensions for a typical 18 watt type chassis on the 18watt website here...
http://18watt.com/modules.php?name=Down ... oad&cid=24
...and there are also chassis and transformer dimensions for Trinity's products in the "Resource" section.
I am currently in the midst of designing my own custom chassis (for a v6) and if I could suggest, do yourself a big favor and either buy a Trinity chassis or the whole kit. I happened to have a specific application where I wanted a custom configuration of the box, otherwise it would have been a complete amp from Trinity. Stephen optimizes and continuously improves his designs and the parts he chooses reflect his attention to this detail. The documentation is great with information for both new builders and for us older, more experienced builders. Don't forget this forum comes free with every purchase! One man's opinion
BTW... Drilling sheet metal can be really bad for guitar players!
Joe G