What thickness steel for mounting plate for bearings to gantry

Hi, I want to make my mounting plate for my gantry now.

I'm attaching the gantry to SBR25UU bearings (At least 2 of them per side) the rail will be mounted sideways on.
Attachment 21487

The gantry will be made of 10cm*5cm heavy duty T slot (Probably with angle steel 3mm on the side with the hiwin rail and ballscrew and underneath the steel will be t nut in for extra strength I will mount the carriages so they don't interfere.

I was wondering what thickness steel should I use to be safe I expect the gantry to weigh a fair bit the supported rail is 25mm so I think weight is not so much an issue for the rail or bearings and I can use 3 bearings if need be (2 are 13cm long) I'm going to cu the rail down but I can leave a bit extra overhang so it doesn't affect my working area if I do use 3 bearings. My z axis is going to be heavy as it's going ot be made from this. I will strip it down and have new supports made from more reasonable sized aluminium but it's still going to be heavy. The ballscrew is a 20mm and ground!, the smooth rods are 25mm with bearings. It weighs like 25kg's at moment hoping to get it down to 10kg with the motor & spindle but we will see.

Attachment 21488

I bought 8 bearings so have spare if weight is an issue.

I will cad this out I was winging it but I think the gantry needs to done in cad so I don't get it wrong.

e2a I want to be able to mill ply and aluminium looking at a 2.2kw water cooled spindle for a start.

Attachment 21497

Sorry for the poor quality rendering my old laptop pc (It's my home pc the work one is much better) doesn't support the quality rendering but starting to cad it out so I can work out how to make the mounting plates (Blue in pic).

Going to work at 4mm steel unless anyone has other suggestions?

Seeing suggestions for 15mm aluminium but if I use steel I would assume I can make it stronger for a considerably thinner mounting plate. Obviously thickness is irrelevant but I'm going to have these plates cnc'd out by a friend on the cheap and he said aluminium or steel makes no odds to him.