IIRC you can get the foam at places like Big Al's (LFS), or off amazon in big sheets/rolls.
I had no issues with silicone/acrylic - just used a lot. Be sure to let it cure for 48-72 hours minimum, most especially if you are really gooping it on. After 24 hours anything thicker than 1/4" will still be liquid on the bottom.
Note that the design you're showing - the first baffle is too high (well, no point in being that high). The water level in that first section will only get as high as the next baffle over.
Also, there's no 'dry' section to this, i.e. for a drip tray. Everything I've read (barrreport, here) indicates that freshwater sumps really benefit from a drip tray so you get more types of bacteria.
For mine, I have a higher first baffle (raised 2" off the bottom) than necessary - aesthetics, but a functional mistake. Note the second baffle (which touches the bottom of the sump) has the drip tray immediately flowing off the same height.
This design allows me to have a wet/dry sump with only two baffles. The drawback is that the entire unit must be sealed to reduce CO2 off-gassing.