Gnerally o-rings are used for sealing endcaps. Sheerline use a custom flat seal, not sure if there is any advantage in those, I've always found o-rings work fine, and they're easy to get hold of. There are two ways of sealing and endcap with an o-ring, radial compression and axial compression.
Radial compression seals on the inside of the tube, the advantage is that the the seal is compressed without any requirment for rods to pinch up the caps. The disadvantage can be the tubing itself- often far from perfectly round on the inside, so you can get leaks if you're not careful. This is especially the case with larger diameter tubing. Examples of commercial WTC's with axial compression are Caswell Sub Drivers and Ron Perrot's piston tank WTC's
Axial compression seals up against the end of the tube. This is usually machined for a nice flat edge. This is how Sheerline and OTW seal their modules, OTW use o-rings, Sheerline do their own thing with seals as mentioned eariler.
You need rods to compress the seals, three is plenty although some use four. To seal the ends of if running internally, you can use o-rings again or flat rubber washers e.g. tap washers. Not that you will need capped nuts to avoid water working it 's way past the seals via capilliary reaction. OTW use very nice custom machined knurled nuts. A cheaper and no less effective alternative is to use brass domed nuts (try screwfix) with washers soldered on the end, which hep spread the load, and give a nice flat surface for the seal to compress against.
For o-rings you can try the following suppliers. Most use either Nitrile or silicone The latter are softer, but a little more fragile.http://www.simplybearings.co.ukhttp://www.polymax.co.uk
I would recommend the following sizes of o-ring seal (o-rings are always specified by internal diameter)-
100mm diameter tubing radial compression- 90mm diameter 3mm thick
100mm diameter tubing axial compression- 95-96mm diameter 3mm thick
120mm diameter tubing radial compression- 108mm diameter 3mm thick
120mm diameter tubing axial compression- 114mm diameter 3mm thick
Note that if you choose radial compression, you will need to machine a groove into the endcap. This groove should be toleranced to allow the o-ring to extrude as it is squeezed, as the rubber will not compress. The amount of squeeze can be from 15-30% of o-ring thickness, I recommend the lower figure, and personally aim for 15-20%. Size to the following for both tube sizes-
Groove width 3.7-4mm, depth 2.1-2.5mm