Fender Deluxe 5E3
The 5E3 is a guitar amplifier, which is still very popular. Significantly more usable than a Champ but technically still manageable, it is a nice DIY project.
Information and diagrams can be found on www.brieskorn.de.
I have drawn a plan for a circuit board and provided the possibility of negative feedback. Also space for a resistor between cathode capacitor and ground. So you can degenerate, for example 47 Ohm / 2.7k like with the Princeton Reverb. This makes the amp a little cleaner. Reconstruction to fixed bias is too expensive for this result, just go a little upwards with the cathode resistor (I think I have 390 ohms with Hammond 291BEX and GZ34).
For the back, it makes sense to make yourself a cutting template. But you can also work with Forstner drill and jigsaw.