I've received the 5mm steel plate, but I'm concerned about the weight.
With the item on the display base, it's very heavy indeed and not ideal.
4mm thick plate would be a little lighter as well as better looking for this project, but then 4mm would present even more of a problem with distortion.
As suggested, I'm thinking of buying a 70mm steel disc to weld to the bottom of the tube and having a countersunk screw go through the bottom of the base into a nut on the inside of the disc.
Is this the better option here and will the 2mm thick tube stay in shape with the disc welded to it?
The tube is a snug fit inside the column that's going over it, so there is no room for the tube to become distorted in any way.
Is it going to be free standing? Why are you so concerned about distortion? Could you use some stick on rubber feet under the plate? If you weld a cap on the end of the tube you just move the problem don’t you? You could weld a piece of bar into the tube inset into the end a little. What finish will you use? The welds might be visible on the outside.
Yes it's free standing. It will be on a shelf on display, so the base plate needs to remain flat.
I probably will fit some little rubber feet to the bottom of the base plate.
There are 70mm diameter washers with a 9mm hole available, so I was thinking it might be better to weld one of these washers to the base of the tube, and then the tube would have a capped end that simply bolts to the base plate.
The finished base is going to be powder coated.
Yeah I'll replace the plate with 4mm, as this 5mm is just too thick and heavy for a display base and then I'll go to the welders and see what they say.
Here is a pic of the base in 5mm