I added an armrest/cupholder to my SX/4. I started with the console shown below. I don't remember who I bought it from but he made a perfect cut where the black line is and I used the rear portion to fashion my cupholder/armrest
Started by making the cup itself out of fiberglass:
I cut the hole in the front part of the console and laid fiberglass cloth on the inside of it. I used a flat tip on a soldering gun to melt the cloth partially into the plastic. I then cut small chips of ABS from a broken grill and melted them in acetone overnight. With the plastic/acetone mix about the consistancy of maple syrup I spread it thinly over the cloth melted to the plastic to make a more solid attachment. Then another layer of cloth melted in with the soldering gun and more ABS dissolved in acetone to completely cover it and so on. You have to put the mix on in thin layers or the surface hardens before the stuff underneath does and you get bubbles in it as the acetone underneath trys to evaporate through the hardened surface.
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After I had 3 or 4 layers of fiberglass cloth bonded to the underside of the ABS plastic I made cuts through the cloth covering the hole so I could use fiberglass resin to bond the cup I made into the console. I slipped the cup through from the top, positioned it and put resin on the loose cloth laying on the cup being careful to keep the resin off the ABS surface of the console as fiberglass does not stick well to ABS. After the cup was bonded in place I filled the underside of the cupholder area with short strand fiberglass. In order to bond the short strand fiberglass to the entire underneath area the last layer of fiberglass cloth I put on was only partially embedded in the ABS with the soldering gun so that the underside of the cloth was sticking to the ABS but the top of the cloth was still clean so the fiberglass could stick to it as seen in the third picture in this post.
I got the console to the point where you see it below and was just going to paint it and my husband said that's mickey mouse, you need to build up the console to be even with the top of the cup. So, I got a can of expanding foam insulation to build up the area around the cup planning to fiberglass overtop but the fiberglass dissolved the hardened foam so it was on to plan B. Plan B was to use expanding foam for autobody use which is put in A Pillars and so on of new cars to stiffen them and provide sound deadening. The autobody expanding foam was almost $100 for a small tube! The fiberglass didn't dissolve that so it was full speed ahead.
I built walls out of cardboard and taped them to the console to contain the expanding foam and it worked well enough:
I used a serated knife to rough in the shape of the foam and then sanded it to finalize the shape:
I then cut fiberglass cloth to cover both the plastic and foam part of the cupholder. On the lower plastic part I again used the soldering gun to partially melt the cloth into the plastic and then dissolved bits of ABS plastic in acetone and put thin layers of that over the lower part of the cupholder as you can see in grey in the picture below. I then put fiberglass resin on the cloth covering the upper foam area (green) once again being careful to keep the resin off the ABS and the ABS off the resin. Several layers of fiberglass cloth took a very long time. Then I sanded down the cup holder area to make it level and a few more times had to build up fiberglass resin and ABS after sanding as I'd sanded down to the cloth and still had low areas.
I had a lot of trouble with the acetone creating bubbles after layering on the liquid ABS. I poked holes in them with a pin, pushed them down, sanded again, more liquid ABS and so on. It took a very long time to get an even covering of ABS. Eventually I got it painted. The console was apparently not out of an Eagle so I had to shave the bottom part of the console to make it fit the Eagle trans tunnel.
I had a local upholstery shop redo the foam armrest in a purple fabric and here is the final result: