Crossbeams are those tubes between the two hulls of  beach cats that keeps all things together. Usually beach cats have only two crossbeams, fore and aft, the fore substain the mast while the aft support the mainshet traveler. Thus crossbeams can be made of composite materials (carbon glass epoxy), i believe that simple (and easy to find) aluminium tubues can be used. This type of material can with very difficoulty lead to a first class failure for a cat like a beam rupture.
Another easy way to make a crossbeam is with wood. Many bigger cats have double T shaped beams made of wood impregnated with epoxy and reinforced on the outside with fiberglass.

I don't know how to calculate the exact properties of this beams (i'm not an engineer), so if out ther is someone who can i wll be very happy to give him all the data needed to calculate this beam. However mine crossbeams could be made of an alluminium tube for the fore(100 mm diameter 2mm thickness 426cm length) but this would be too prone to be damaged i believe or a 130mm diameter 3mm thicness for an alluminium fore beam, consider for example that the RC 27 has two tubes of aluminum 6061-T6 of 150mm diameter 3mm in thickness. This is high strength alumium but also it seem to me to be a little expensive, at a very first look i have found that a tube like this cost something 2,3 US dollars per each cm, but i have to look better. It should be interesting at these prices (provided they are correct) considering the use of carbon epoxy tubes. For the rear beam as it sustains fewer loads, it could be possible to use an aluminium tube with the same measures of fore steel beam. 

Here it is a easy .gif image that shows well the beam loads

Moreover the fore tube can be stiffened with a

v shape stainless steel strip that is called "dolphin striker???" whose apex substain the mast thanks to a little beam of steel that goes into the crossbeam and beyond. So this becomes the mast foot. The dolphin striker allows to transform the bending load on the main beam due to the mast compression in a more workable compressive load on the main beam.

Crossbeams can be directly screwed into their hull's slot( in little cats) or more properly can be hold in place by two strip of curved steel around the beam that are screwed to their four slot. It's usefoul to reinforce that part of the tubes that goes under the weight of screw or mast foot with a piece of tubes put vertically and cutted to fit exactly the internal diameter of the tubes.