++1 for Plastruct. Since you are going to build catenary for looks only, the supports will have only the relatively light weight of the cat wire in a vertical direction, with no appreciable horizontal forces other the the errant pantograph or sweater sleeve; thus you don't need the rock solid rigidity that tensioned overhead requires.
Assembling your own rather than accepting mass produced ones, either brass or printed, gives you a lot more flexibility. For example, since you are I believe a three rail person, you may use / need a track spacing on curves > the 4" with the brass ones -- I think 4.5" is common for 3R. You can also increase the overall width where required -- to allow for station platforms, for example. I would build a jig that allows you to vary the width of an assembly on the workbench, build it there, and then transport it to its mounting location. [ I used this approach to build wire cross spans, and move the jig with towers right to the installation site.]
You can use a variation of the verticals through the baseboard: drill the hole so that the H beam fits through snugly, but not a force fit, and mark it; you can then remove it and drill holes to insert nails horizontally through the web above and below the surface; alternately, you could screw it underneath to a "pillow block" of wood -- or some other solution.
I would put small L braces [ of P'struct angle ] underneath the attachment points of the crossbar to the verticals, to provide additional 'glue' locations, rather than just the cross beam end. I think I would, if the size shapes I would like to use were available, use ABS rather than styrene, or at least see which worked better, both for assembly [ should be the same ? ] and durability [ ie, can you bump into it and have it survive; ABS should be better....].
I'm not sure how I would simulate the concrete bases, but I can think of a number of approaches, so you should be able to, too. Which reminds me: read and re-read the John Sethian articles referenced above for his experiences.
I have not built complete catenary supports, but I have built a fair number of the horizontal pieces, such as cantilever arms for two tracks ( usually attached to S'feldt towers ), using Plastruct shapes, and have been pleased with the result.
I hope the above gives you some ideas, preferably usable ones...
Best, SZ