AV11 just isn't viable for a combat vehicle. It's barely acceptable for a transport (the Trukk gets by because the Ramshackle rule makes it safer).
Sure, the Boomgun is nice, but you've got glass sledgehammer syndrome.
Trust me, I've fielded Razorback heavy SM lists. An AV11 tank with a big gun usually becomes a smoking wreck midway through turn two.
If Looted Wagons were a Dedicated Transport option, or if you got Three per FOC slot like the old days, then they might be worth it...but one per FOC slot? I don;t think so.
Especially since the Boomgun is the only thing that makes it worthwhile. Now you;re paying 115 for an AV11 tank that had just enough firepower to become everyone's primary target.
As for making a looted Rhino or Chimera look Fast and Open-Topped, that's easy.
For a start, if you're lazy you can always just buy an 'Ard Case for the trukk and make it closed-topped.
If you want it to stay open-topped, all that requires is a hobby saw. The top o the chimara comes off anyway, so just leave that bit off. Replace the hull-mount heavy bolter with a big shoota...or just seal it up and put the shoota in the turret (the hull mount slot would be a good place for a stikkbomm chukka)
For the Rhino, just cut away from the top hatches back and leave the track guards on. Now the cockpit is closed off (makes life easier since there's no detail inside there) and the transport bay is wide open. Stick an Ork with a Big Shoota out of one of the top hatches and you're good to go.
To make them look Fast, you just stick on some huge freaking exhaust stacks. Bolt some half-barrels to the sides for extra fuel (also helps to sell the weaker armor by putting vital components on the outside)
Take off the side access ramps from the rhino and cut an engine block (from a car model, or from the trukk model) in half. stick each half against the inner door of the side hatches.
And flame decals go a long way towards making anything look fast ;-)
In both cases, bolting on patches and scraps of wood show the weakened armor. If I can stick three rhinos together and make a battlewagon out of them
, I'm fairly confident I can turn a single Rhino into a Trukk without breaking a sweat.
You just have to be creative.