I have a different method.
A while back, I purchased some sheets of Tufnol (aka Bakelite) of various thicknesses. It's useful stuff for making jigs and gizmos.
First I cut a backing plate from 10mm thick sheet, about 10mm larger all round than the stompbox (I tend to stick with one size of box where I can, so I only need the one backing plate).
Placing the box lid in the middle of the plate, I then marked the positions of the fixing holes, and drilled them large enough to clear the lid fixing screws. On the back of the plate, I counter-bored to allow the screw heads to sit below the surface, so the plate wouldn't rock when the box is being drilled.
Additionally, I mounted a thicker block of Tufnol onto one of the short ends of the plate, in such a way that, with the plate 'on its back', the block was flush with the bottom edge.
All this could have been done in wood, I suppose, but the Tufnol will keep its shape and last a lot longer.
That's the hard part done - and it only ever needs doing once. Now to drill the box - having already marked out and punched where the holes are to go.
I mount my box, without its lid, onto the backing plate using the lid screws to secure the box to the plate. The assembly can then be held, by hand (as I use a pillar drill), and the top face of the box drilled. If I were using a hand-held drill, however, I would hold the backing plate in a bench vice. It is thick enough not to buckle when the vice is tightened.
The extra 10mm width of the backing plate (ie, 10mm larger all round than my box) means that the assembly can be turned on its side, so the jack holes can be drilled without the lower side of the box getting scratched by swarfe.
Finally, the assembly can then be stood on end (using the end block) so the hole for the power connector can be drilled in the top end of the box. Et voila.
The time invested in making the mounting jig was well invested and, like I said, only needed doing once. My boxes can now be drilled without scratches, and without any risk of vice jaw marks. Seemples !
”Sex is great - but you can’t beat the real thing !” - The Wanker’s Handbook