Use dry block heaters to heat samples evenly without exposing them directly to the heating element. Also known as dry baths, they don't require water or other bath fluids, which must be periodically replenished and can contaminate samples.
These units use modular blocks to hold samples as they're heated. The blocks are interchangeable to accommodate a range of lab tube sizes.
Change the modular blocks in your dry block heaters to hold different sizes of lab tubes.