Are the welds both on the inside corner and the outside and highlighted in yellow below? Yes, the welding is on the entire cross section of the aluminum profile, on the outside, inside, top and bottom (see figure).
As for the location of the screws, they pass all the aluminum profiles and the rubbers as shown in the figure, from the steel plate.
To go from the solid tubes to a sheet body with only four faces, I can do it transforming the ones I already drew, or I must do it from scratch (I don't have much experience).
I remain pending to the part of the soil.
Thank you very much for helping me so much and for the recommendations you make about the model in general.