Ordinary plastic is fine. You can buy concentrated H2SO4 at an auto supply store in a plastic bottle.
Edit: maybe not fine. Resistance to H2SO4 corrosion falls off sharply as its concentration approaches 100%. On Venus, the clouds would be very nearly 100%.
At low concentration a sulfuric acid solution corrodes only by being an acid.
Neither carbon fiber is affected by acids, nor some types of plastic with carbon-carbon bonds, like polyethylene (but polyesters and other polymers made by polycondensation may be hydrolyzed by acids).
At high concentrations, a sulfuric acid solution begins to have an oxidizing behavior, even if not so strong as nitric acid, so it can convert the carbon from carbon fibers or some plastics into carbon dioxide, damaging them.
The difference in behavior is because the sulfate ions have a very high affinity for water. At low concentrations, they are strongly hydrated, so the attached water shields them from making direct contact with an immersed material. At high concentrations, there is much less water available for hydration and the sulfate ions can make direct contact with an immersed material. Then the oxidized sulfur atoms from the sulfate ions can oxidize any less electronegative elements.
That was fascinating. As a non- chemist it's easy to think the water is inert and 50% concentration just means 50% as much corrosion, but the water obviously plays an active role.
A near 100% H2SO4 cloud would be a cloud whose droplets have vanishingly little water in them.
As your balloon drifts up or down through it, the sheen of moisture it collects on its surface is maximum strength sulfuric acid. I hope your balloon is not of a material subject to oxidation, because if it is, you will soon discover a place even worse than in that cloud.
