Another great project!

Years back I made a 1/10 model of the Hetzer. I actually had purchased the basic pieces from a guy in Germany who started the project.
He used 2mm (~.080") thick aluminum sheet to make the upper and lower hulls. He had glued most it all together and hadn't got much further.
Fastening those hull plates and keeping the correct angles was a challenge

My solution was to make short small sections of similar aluminum sheet formed to keep the upper hull plates at the right angles then using flush
rivets to fasten the hull together. I am unable to weld aluminum or I would have tried in but thin sheets or anything can warp when heated so
I'd recommend something similar to the riveting technique I use. If you are a welder or have access to a pro welder go that route to make the upper hull.
Concealed flush rivets make a strong hull. I see you've already cut your upper hull plates or I would have suggested to make small 'tabs' on the sides that
join so you could use those to rivet to.

Some time after build my Hetzer I found another 1/10 scale Hetzer that was very nicely welded from steel albeit thicker than my aluminum version but
they managed not to warp the sheets by do shorter nearly 'tack' welds.

Jerry