I would like to share the results of my DIY espresso lever, built with minimal use of power tools or fabrication of custom pieces. This is not a boiler, it's just a piston with a lever, the water needs to be poured from the kettle. I was not trying to get a professional espresso quality by any means, nor was I certain of whether the pressure was going to be enough (it sort of wasn't in the end). I learned a few things in the process, so I would definitely change a few thing if I were to build another one. I don't really feel the need for it though, as I'm satisfied with the quality of the coffee. One of the biggest failures was with the original idea I had of creating a non-return valve system that would allow pouring the water without having to lift the piston out of the cylinder. It is probably possible to do, but I couldn't make it work by simply using inexpensive and readily available parts such as silicone sheets and washers.
As I mentioned above, the idea was to build something that did not require any special or extremely expensive piece. In fact, the only pieces that are espresso-specific are the filter basket, the gasket and the shower screen. The main part, which motivated me to build the piston in the first place, is the stainless steel sanitary spool with the tri-clamp ferrule. The diameter of the ferrule flange for a 2'' pipe matched almost perfectly the OD of the a 53mm basket rim (flange was 64mm and basket was 63mm). The thickness of the basket rim was also adequate to provide decent compression when tightening the tri-clamp. Between the pipe and the basket I used a 2mm silicone gasket from a 3-cup moka pot and used its filter as the shower screen. The piston is a combination of discs, bespoke silicone gaskets (48 mm dia.), hex bolts, threaded sleeves and lifting eye bolts. All the metal parts are stainless steel, of course. I don't think I need to explain more, the photos should be clear enough by themselves.
One more think worth mentioning, if anybody ever wants to do something similar using a sanitary pipe, is that it would be much better to use a 1.75'' pipe instead of a 2'' one. A 1.75'' spool uses the same flange diameter, but you can apply much more pressure with it. I don't remember exactly what the ID was for a 1.75'' pipe, but the increase in pressure should be at least of 30%, plus is easier to find steel discs with smaller diameters for the piston (like penny washers). I was able to find almost all items on ebay and Ikea, except the steel discs for the piston, and probably spent about £40 getting all the parts.