I started out using MDF with an aluminium tube frame, worst thing I did especially for a largish layout, while that was larger than yours, when I realised even the smaller parts with the aluminium was not stable I got rid of it when we moved house.I'm curious what sort of tube (and connection methods) you were using? I've just completed two aluminium cart frames, so I'd like to know what issues you had before I proceed to go all in on aluminium framing.
Sorry for late reply, what I found with the aluminium square tubing was that over distances you need quite a few cross braces, much depending on both the length and width, if one stuck to the standard no structural pine of 2440x1220x13mm ply, just using metal screws to secure the ply to the tubes did not give enough strength to pull the tubes up onto the boards underneath was insufficient to hold it tight, especially when the black heavy duty joiners of different styles, that included simple T's, 4way crosses and 6 way cross and supports. I did try short legs at one time under the main frame and they worked loose whether with wheel in the bottom plug or a shorter stand, all of the sections tended to wobble, and more you added and cut sections the worse it became.
In the end it became fairly expensive and for me unstable when compared to the 70x35mm non structural pine legs with the base board support on the top with smaller cross supports around every 1m and same size supports along the outside of the frame with 38x19 pine. Secure screwing holds all in place and have the legs at ends & one or two in the middle, have lower leg cross piece on around 150 - 200mm off the floor to hold it square. The extra supports can be added later if its a permanent layout. Be careful though re the height of the layout, the higher, more cross supports and checking them for being secure and not wobble.
I use 50mm 1gm wood screws, not of the brass types, and its a lot easier then screwing into the tubes.
There has been some articles on building the aluminium frames and think one is in the current edition of AJRM magazine will check in the morning a reply, but there is a lot more in the way they are constructed today. I would suggest that perhaps the big issue may be connected with the aspect is the framing for a portable layout eg: for exhibition layout, or a permanent home layout.