How about a tank destroyer type with fixed main gun...i.e. STUG type
They built a STUG type self-propelled gun on the M3 chassis, the T18 GMC:
The open-topped T82 105mm was also built:
... and pre-war the 75mm T3 on the M1A1 Combat Car chassis:
Also (pedant mode on) the STUG (Sturmgeschütze) were
self-propelled assault guns, not purpose designed
'tank-destroyers' (Jagdpanzer), some models were used in that role when necessary but that was not the intent.
In the STUGs the gun was mounted toward the center of the vehicle, not forward like the Jagdpanzer and Hetzer,
(Jagdpanther is its own, unique animal).
Note that on the T18 the gun is roughly in the center fore-and-aft, and offset to the right. The small size
(if you ever get a chance to climb around one, do it, they are incredibly tight inside) of the Stuart and the
arrangement of the drive-train (mid rear-engine/front-drive) and short hull gives a designer few options,
a cutaway that shows the problems:
In the non-turreted 3" TD designs built, T56 and T57, the engine was moved forward and the gun mounted
to the rear.
T56
T57
While based on the 38(t), the Hetzer had a redesigned and enlarged hull, a Stuart based 'Hetzer-ish'
jagdpanzer, would also need an enlarged hull, in particular it would need to be longer. Also a pancake
engine wouldn't go amiss.
My own long stalled M3A3 with Vulcan:
It has been painted, but I just realized I have no newer pics of it, I'll get a couple and post later.
In closing, the M3A3 is by far the best looking of the family, the raised rear deck of the M5 just looks kluged.