My recommendation would be a tuniq-tower style HS with rows of parallel tubes creating a rectangular area to replace the plates.
A though occurs, tho. It is possible to fit more plates into a given volume than cylinders. My optimization was based on a constant volume of material - so given say 10 cm^3 of copper tubes would be the best design. What may be more useful would be to find the shape that maximizes surface area within a volume of space. Back to calc...
P.S. - Just in case anybody was planning on actually creating a design my previous optimizations found that
1 - given a volume V of material taking the form of a square plate length x and depth y the optimal (most surface area) dimension is x = V^(1/3)
2 - given a volume V of material taking the form of a cylinder with radius r and length x the optimal dimension for r is given by r = V^(1/3)
Good stuff there!!! A flattened cylinder might be a Happy compromize.....