Christian McArdle wrote:
121 cm. Back edge and sides well supported.

Then 20mm x40mm with 3mm wall will be fine - use 2 lengths - front and
middle.


Obviously the 20mm is in the important vertical direction.

I gathered that from your limited space.


Also, the metal
companies seem keen of "16" gauge, which they suggest is 1.5mm, rather than
3mm. Is this well underspecced? Does anyone publish formulae?

I wouldn't use 1.5mm wall for this.

I think you'll find it difficult to get a calculation on this - because
what you're aiming for is as close to zero deflection as possible. This
isn't normally calculated for. There has to be a certain amount of gut
feel/experience here. If you play around with some box section, you'll
soon see what I mean.

3mm walled box is easy to get hold of.

--
Grunff