I should add a few notes about the photo of mine from above...
Inside is a copper t-piece with lengths of copper pipe about 1ft long, drilled with little holes along the top surface.
I assumed this would displace the steam more quickly and evenly around the wooden box. Now, when you open the box, it's pretty damn foggy in there, but I honestly have no idea if the pipework inside makes it better/quicker/easier or not, I never tried a 'before' test without it.
In the sides, along the joint of the two pieces of wood, I banged a row of 3'' nails (one row on either side) to support the wood during the steaming process.
Worth noting that where the wood rested on the nails, it did mark the timber I was steaming (not a problem for me as the marks almost sanded out and were inside an instrument with a tiny soundhole anyway, never to be seen again).
Next time I use it, I'll replace the nails with small dowels or bamboo skewers.
Again, I assumed raising my piece of timber off the bottom of the steam box would be ''more better'' and I had to do it anyway because of the copper pipe I'd installed in there.
My steam generator: Nothing I would improve or change there, it worked perfectly. Gas burner / pressure cooker with the valve removed / washing machine hose.
There's a washing machine tap visible, I installed it to keep the steam out of the box until it was properly mental boiling, Again, I have no idea if it's necessary.
Just a few notes from having been there, done that. Albeit infrequently.
Try it, if it ain't working then try summat else, it's all a laugh, innit?
Oooh, important addendum: Don't write 'THAT' on it if you have children that use the internet.
Nice. Thanks for the info. How long is the box?