My first amp was a DIY. My kuya had a spare car stereo speaker. What I did was I asked the carpenter who was doing repairs on our house to spare me some recycled plywood and make me an open-back cabinet.
Then, I went to Raon and bought a 10 watt IC type amplifier, some power supply parts (tranny, rectifiers, cap and plug) and a microphone preamp. Tied them together and ta-da. All in my out of pocket costs were very small, about P200 at the time (that was in the 80's). It worked and we were able to use it in practices here at home, along with some other DIY gear like my first distortion, flanger/chorus boxes (alexan and tanioka).
In hindsight, a microphone preamp is not suited for guitar as its typical input is 600 ohms...too low for the impedance of guitars. If you wanna do that, just pull out the input shunt resistor and replace it with a 1 Megohm resistor.