I am not clear on what 30 x 50 is? Is that centimeters or inches?
30 inches by 50 inches is plenty of space for two guineas.
30 centimeters by 50 centimeters is only 11.8" X 19.7", so in that case the cage is much too small for even one pig, much less 2.
I don't know what State or Country you are in, so I can't help much. If you can post a general idea of where you are, we may can help you find CC supplies.
If you cannot find coroplast or grids anywhere, you can also try linoleum flooring, a thick plastic or rubber mat, or a shower curtain for cage flooring, and either wire or closet shelving for cage sides/walls. The CC Cage idea is pretty adaptable, you just have to use your imagination of things you can use/find. There are lots of ideas in the "alternative cages gallery, in the photo galleries.
There have been some people here who have used the kid pools, and it would be enough space for 2 pigs, but you would probably still need some sort of wire around the sides. They are called wading pools, or hard shell pools here.
The ones like this:
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A snap set pool like this, might also be workable as a last restort:
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As for giving your pigs things to do. Mine love boxes with holes cut in them, pvc pipe, piles of hay, just lots of things to explore through and play in.