The picture shows the 'Mark I" lid. All the discs were roughly cut with a jigsaw (you have to keep going or it will melt and reseal) and finished with increasingly finer grades of wet sand paper (the black stuff) If you can get access to hole saws and a drill press life will be so much easier. The main top part is 1/2 ' acrylic and the spigot is 8mm, with a decent 'O'ring you will want at least 8mm for the spigot, if I was doing it again I'd go probably 10mm. Mounting the toggle switch has its own set of problems, you want enough of the stem coming through the top to allow the waterproof boot to screw down firmly on the acrylic to make a secure seal. You also want to be able to replace the switch, if ever necessary, without destroying your lid. I opted to cut a hole right through the spigot and the main top piece and then laminated another 6mm thick disk on top. The switch screws into a tapped hole in the 6mm piece and the hole in the bottom two layers allows the switch body to rotate freely for installation and removal. The other option is to rout a recess in the main top bit that allows the switch to come up further, the problem is making a good job of this that allows the switch to fit flush.
you want a simple toggle switch rated for 10A DC, remember you will be burning 4.2 amps on a 50 W and if you wanted to use a 100W for a little while you'd be pushing more than 8 amps. It never hurts to have over rated components. I used the 10A toggle switch from Maplins. They come in different types but all you need is a single pole with simple on off action. (see components list). The key thing in relation to your lid is that this particular switch has a body that requires a 32mm hole to rotate freely and a stem that needs a 10.5mm hole tapped for M12 on the top mounting piece. If you use 6mm for the mounting piece the stem will come through enough to take a standard cylinder 'O'ring underneath the waterproof boot which helps make the seal. The other hole you need is for the cable gland which allows your 'umbilical cord' to leave your canister and stops water getting in, ingenious.
you will also note the 'strikes'these are little stainless catches for the compression latch system. I bought mine from Mcmaster-Carr but I have since sourced similar items from RS components.