Regular neoprene O-ring material should be fine for most coolants and you can get it in cord various sizes.
You can often get away without joining the ends; just cut the cord slightly longer than the groove length and butt them together. If you want to glue them use a flexible crazy glue; the O-ring cord supplier probably has the type needed.
As Cartierusm mentioned make the grooves slightly shallow so the O-ring cord is compressed. In our experience 3/4 of the diameter may not be quite deep enough depending on the area under vacuum and the thickness of the part because it does take quite a bit of force to compress the cord. You may need to experiment a bit. Get a soft grade of Neoprene, 70 Durometer I think may be the softest available, as this allows for more compression. Also make the groove 20 to 30% wider than the diameter of the cord so it has room to expand sideways when it is compressed; use the measured diameter not the nominal as O-ring cord is oversize slightly.
An open mind is a virtue...so long as all the common sense has not leaked out.