I got a 1x2x7 piece of maple from the Despot and cut it into appropriate lengths. You may wonder about the 7 foot length - my piece probably started life as an 8 foot section, but someone cut a bit off. Not a problem as 7 feet was more than what I needed.
An aside: my piece of maple actually started life as a tree...but then became a board. It actually has some nice flame to it - I went through about a dozen pieces and found a good one.
Hopefully this picture better illustrates what I'm aiming for. I measured and cut one piece, and mitred those ends so the 'short' side (e.g. the inner side) would line up with the long side of the chassis. Then I measured and did that for the other 3 sides.
I discovered something during this exercise: I found out that my $99 mitre saw isn't the world's most accurate. For some reason, the mitre angles wouldn't line up when I cut them at an indicated 45 degrees on the saw! With some trial and error, I did wind up with 4 pieces that worked. I wound up with just enough wood out of my 7 foot section to get 4 sides.
I then glued the sides together to form a 'box' around the chassis. Since I have only one long clamp, I used rubber bands and the power transformer as makeshift clamps. Note to self: get more clamps.
I also realized I'd need some sort of feet on the bottom of the amp so it wouldn't just lay flat. I originally just envisioned rubber feet, but then I had an actual flash of inspiration: round feet cut from the maple scrap!
I used a 3/4 hole saw in the drill press to cut the feet. Now, if you don't know this already, maple is a hardwood. This means you must go slowly when you cut it. I found if I rushed the cut, the wood (and the drill bit) would get so hot the wood would (ha ha wood would!) start to burn!
So working very slowly, I cut the feet - it took over 10 minutes to drill each one. But it was worth it I think.
After the frame was glued up, I drilled holes from the inside for attaching the chassis. I previously drilled holes in the sides of the chassis as a mount.
I lined the chassis back up in the frame and marked the holes. Since I couldn't get it under the drill press or use a hand drill, I used my Dremel right-angle adaptor to drill pilot holes. I've had the adaptor for a while (it came as a freebie with my Dremel), but I've never used it. It worked a charm on this job.
Again, the maple is hard and there's no way you can run screws into it without a pilot hole. This is one place where I planned ahead and it paid off later.
With the holes drilled, I then glued the legs to the bottom of the frame.
One thing I would do a bit differently is the actual fit of the chassis into the wood frame. I had to take the chassis in and out several times, and it was a very tight fit. So tight, in fact, that I dropped the frame and the corner joints came apart! I had to reglue it.
I think in my really serious amp build (with the 2A3s), I'll shave some wood off the inside of the frame somehow so the chassis will slide in a bit easier. This I shall ponder.
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