Finally got a video of the Voice Wobbulator. Maybe seeing the design in action will help. Originally built from the 1st pre-development SSR model. 1st relay gets its power and is controlled by the wiring harness and controls. 2nd relay gets its power directly from the 556 IC. Not ideal at the moment. But the design works.     Forgot to add some photos to the previous post.   And these for the future quad oscillator design concept.    

@RepeatedMeme it's Kingspan insulation board, the yellow stuff. If you peel off the foil layer and sand lightly all over (to open the cells) then polyester resin just soaks in, no melting or dissolving.   I'll mould it eventually for any future builds, but it's sturdy enough to stand on even now. Anyone going this route can go one further than me and fibreglass the underside too, for even more strength. But I just used a piece of 12mm ply cut to shape, which the black trim can be stapled to later.

Interesting approach with the fender top. It works really well.   I don't think you'd mentioned it before.   What sort of insulation board is it?

The base: So previously in this thread I shaped a fender out of 2" insulation board. My next step was to laminate a thin layer (about 300gsm) of matting over the surface, rough sanded the peaks, and spread filler into the troughs.     Once it was about P60 smooth, I mixed up a highly pigmented red gelcoat, and painted the lot. This'll help match it up with the rest. I'll wet sand it with some fine grit tomorrow, so get rid of the streaks.     Just the hemis to go, apart from a couple of small finishy bits. They're all cast now, just need glassing in. I'm hoping to get it ready for an event at the weekend!