I've been building Flite Test planes ever since it was Chad and two Joshes in Chad's garage and me in my shed, and although the later planes are great, planes like the Spitfire, Mustang, Versa and Racer are still the ones I keep flying.
They were designed at the time when the powerpod was planned to be a standard part that you could take from one plane and plug straight into another, keeping the costs down. But I doubt whether many people actually did that - in my case at least, I built a pod for each plane and that was fine.
And in theory, the powerpod design was a sound one, forming a closed box that the battery, ESC and receiver would fit in. But that also made it fiddly in the field, because you had to remove the centre BBQ skewer, slide the powerpod out, put the battery in and reassemble everything, often in a tight fuselage and usually getting some wire or other trapped in the tab slots. And if the CG was wrong, you had had to take it all apart again to move the battery.
So a couple of years ago I designed the inverted powerpod. It's a direct replacement for the normal one only the box is no longer closed allowing you to Velcro the battery in place without having to remove the powerpod. I suspect you might be thinking, 'But the battery will only be held in by the Velcro and it will fall out.'
It doesn't. I have never had one fall out yet and neither have my flying chums who have adopted the design.
If you are really concerned, you can fit a strap for the battery that conveniently passes through the tab holes (see later).
OK, on with the build
Print it out and temporarily glue it to the foamboard, cut the score cuts on the red lines and all the way through on the black ones. There will be four thin pieces at the tops of the tabs to remove and discard.
Remove the foamboard from the slots in the usual way and using A folds, glue it together. The tabs will pop through to the top, so you can't use the table as your friend because they get in the way. Just do your best.
Reinforce the tabs and the BBQ skewer hole with parcel tape.
And glue the firewall on the end making sure the two holes are sticking up.
Wrap the powerpod in parcel tape and lay down a strip inside to give the Velcro something solid to stick to. If you do feel the need to add a strap for the battery, simply poke it through whichever pair of tab holes work best for the CG (where the little thin pieces came from).
All done. You can Velcro or glue the receiver and ESC in place to suit and once the powerpod is in place in the fuselage, you never have to remove it again, which goes a long way towards keeping everything solid (ie the BBQ skewer hole doesn't keep getting larger and larger, and sloppier and sloppier over time).