Are you asking about differential expansion? If so, I think you might be over thinking things. however you can allow for it if you wish, by keeping in mind how you wish to constrain the aluminum. Much like making table tops. constrain and also alow for expansion, the bread board table top comes to mind. however it should not be a problem. temper deviation relative to coefficients  of expansion are going to be small , in the world of wood working. post pics of your work. It will be great to see how it unfolds.

thank you both for the responses.  Overthinking it is prob the best response ha. 

My kids are all leaving me and now I have this void to fill of what to do with my time.  No better time to fill that void by getting into woodworking.  With that said I don't have a table saw and because of limited space I was going to do a larger MFT style table and use FS Track Saw as my table saw (certain limitations and inefficacies with that decision).

My plans was for a 4x8 (roughly) simple base.  Then have a wood framed top that would MDF MFT style table top to be inserted at top.    My thoughts was to purchase (3) Festool replacement MDF perforated tops to mount to top.  That way if ever one gets cut up to a point of needing replacement I could always swap the bad one down to the back side and move a fresher one to the cross cut area (having 3 to swap around) vs. always having to replace the full 4x8 sheet.  Albeit more expensive then purchasing a single 4x8 sheet, i wouldn't have to invest in the tools to do my own dog holes and ROI on my own time doing it.  Then use the FS Aluminum rails that you can purchase separately and mount to the outside of the wood frame.  That is what i was concerned with as I haven't seen anyone do that as an example.  Will get pictures eventually when done, but will be sometime as i as I don't have heat in the garage yet ha.

I have looked into the 8020 and still might go that route, just need to do some investigative work.  Lastly to NVAl comment on Plywood, I might consider laminating some plywood together for the edges which then takes away concern of any movement where the rails would be mounted to.

I agree with @gunnyr on a Paulk or similar style bench.  A 4x8 sheet of MDF wrapped in 8020 would be a bear to move, and would probably have a lot of sagging issues over time without any additional bracing.  If space is an issue (based on the "no room for a table saw" comment), something that breaks down and is portable seems like a better solution than a large, unwieldy sheet.

appreciate you all.  First to admit that I overthink things a lot.  Will review the Paulk style bench and things thru. 

As well agree on the if you have limited space, but doing a 4x8 WB then I can fit a TableSaw.  Very true, in my mind I was thinking I wanted both (TS & large bench) so therefore I would stick with the WB. 

All good points/info you brought up and appreciate that.  Will continue the journey.

@Carlito0822, please know that there is not really a wrong answer.  What works for others may not be your best solution.  I'll second @tsmi243; you'll build several versions so for now just build something. 

I started with a 4x4 sheet of plywood on sawhorses.  My second 'bench' was a 3x5 scrap torsion box that I used for 25 years and is now my son's workbench.  Now I have a collection of assembly tables, MFT's, Paulk, and Bora Centipede work surfaces.  I even have a bonafide workbench!  One of these days I'll need to build something IN my shop to justify the work ON it!