Hey Yankee Tim!
I got my handles, grill cloth and jack port at Speaker Builders Supply. Great folks to deal with. I couldn't be happier with the handles. Dead centre in the vertical axis and 4" +/- below the top of the cab. Perfect for moving this baby. The flat black is a sweet touch.
I used 4 coats of linseed oil with a light sanding in-between. In no time my cab got a natural relic from the many gigs its been to in the last year. I copped a HT plan off the internet. It had enough info to get the basic dimensions to something I was happy with. Not an exact reproduction of anything. I used 1/2" birch ply. Worked just fine. 3/4" would weigh a ton.
Do the handles effect tone.? Well if you have ears like a dolphin or dog, perhaps you could notice the effect standing behind the cab, but honestly, if you are after that amount of detail, well god help you on your tone quest!
I could not imagine handling this cab with the 2 JBL's without handles of some sort, and the one's I picked have been perfect for the ergonomics of handling gear.