You can also
router or rip on the table saw concaves or
convex curves in making large
molding sections out of 2" and wider "extruded" polystyrene foam like Blue Dow board. Not the white dot "expanded" polystyrene.
In routering you need a variable speed
router. On the table saw, it's a specific technique that requires someone trained in doing so to set it up. You can also use various surform and sanding tools to shape it.
In order for it to stick to the counter or itself in building up layers you either need some contact cement designed for work with foam or some Foam Fast spray adhesive. Liquid nails and panel adhesive might or might not work, often not.
Along the same
line would be to use some spray foam such as Great Stuff on the
edge of the counter top - especially if within say a rain gutter form that's cut to fit around the counter, than either leave the gutter in place, or if a
release method for doing molds is used, to remove the gutter form and further carve or refine the mold. You can also do this with just a U-Shaped lumber form for the spray foam to dry.
Carving the foam as above would require breathing protection and probably a anti-static suit, but it can also be done with belt sanders, saws, routers sur-form tools, grinders with chain saw blades or
wire wheels once dry. Still just spray foaming a rain gutter into place would provide a decorative
edge and one that's easy enough to paint in a wood graining technique. The foam would remove that tinny sound when it was touched and prevent it from denting. Also The gutter would be something that's cheap, very damage resistant and pre-shaped as if a decorative
molding.
Another option is to do so in building up lumber that's routered and sawed to make up such
molding. Perhaps even a L-Section with smaller cheaper
molding you make or acquire attached and built up from it.
As with the
PVC tubing, painting foam would be the most difficult part. On
PVC, either use a primer designed to stick to
PVC than paint it or some contact cement that sticks to plastic, than after dry, paint atop it as if a primer. Kilz as a brand of primer might work well as long as it works with the latex paint.
In the case of foam, it both does not take paint well and damages easily. Covering it is necessary with if possible
muslin in making a fiberglass like surface that both takes paint well and is damage resistant. The heavier the weight of the fabric, the better it will adhere and take paint, much less the more depth it will have, but the closer the audience to the prop, probably the better a tighter weave lesser weight fabric will be even if less damage resistant.
Flex Glue or Sobo and other white glues designed better than the standard Elmers should help the fabric stick in a
dutchman like way to the foam especially when the foam is prepped by contact cement, primer or other means. Another option is more in the
line of plaster based coverings like Gesso, there is one type of this I forget about as a product name which is a tuff shell you paint on. Could be something like Tuff Coat.
Down and dirty would be to
gaffers tape the foam than paint it, but be prepaired for it to peel up and be less damage resistance either during the show or in long term storage.
Hope it helps.