The panel was not flat, but I guess it comes pretty natural after glueing up to cupped boards. At first I traversed it with the scrub plane, and then when it was flattish I started going with the grain.
After a bit of workout, it was nice and flat.
I dry assembled the carcase of the chest, and after checking that it was square, I traced the outline of the bottom onto the bottom panel.
According to my sketch it will look good if the bottom is protruding on all sides, just something like 1/4". This could technically be achieved by adding a small piece of moulding, but I didn't bring any moulding planes with me, so I had to think of another way to make this small detail.
The plan was to make a groove near the bottom of the carcase, to house the bottom, That is pretty traditional.
In order for the lower part of the bottom to protrude I had to make a rabbet along all the sides of the bottom.
Then on the remaining full thickness part of the bottom I had to make a groove, so I would end up with a thin piece of bottom that would fit into the grooves of the carcase sides.
In my mind this worked very well.
Design flaw No 1
I normally stick to the grooving before dovetailing, but I didn't do it this time because my original idea was to simply make a rabbet and then nail the bottom on.
Due to my interlocking groove idea, I had to make a set of grooves without messing up the dovetails, so I wouldn't get a groove that would be visible from the outside of the chest.
On the sides it worked really well, since I just cleared the tails.
On the end boards I found myself needing to make a stopped groove - as in stopped in both ends. I tried using my grooving plane, but it pretty quickly became evident, that this tool was not intended for making that kind of grooves.
My solution was to use a utility knife for defining the sides of the groove. I adjusted the iron of the grooving plane so it protruded a little less than 1/4" (5 mm to be exact). That made it possible for me to use the grooving plane as sort of a router plane.
It took a bit more time than I had expected, but the grooves turned out well.
Design flaw No 2
If you need to make a groove on the side of your rabbet, be sure not to make the rabbet wider than your grooving plane can handle..
This flaw was discovered after I had finished planing the almost 3/4" wide rabbet all the way around the bottom.
I think that I might be able to tackle it sort of the same way I did with the stopped grooves. But I decided to call it a day and not risking to mess up things by keeping on working too late.
After imposing those additional challenges on myself, I would like to explain why I changed the design in the first place:
I wanted to make sure that there would be no crack between the sides and the bottom if it should move with the seasons. Therefore I got the idea of housing the bottom in a groove.
And making one bottom, and then glueing on a decorative bottom seemed like an idea that required too much work. Guess I was wrong about that.
Flattening the bottom.
A stopped groove.
Note the protrusion of the iron on the plane.
The bottom with rabbets.